The Complete VoIP Glossary: Every Term Your Business Needs to Know
SIP, PRI, PSTN, QoS — telecom is drowning in acronyms. Here’s a plain-language glossary of every VoIP and phone system term that actually matters.
Why This Exists
Telecom has a jargon problem. Every vendor throws around three-letter acronyms like confetti, and half the time they’re using the same term to mean different things. If you’ve ever nodded along during a sales pitch while quietly wondering what a “SIP trunk” is, you’re not alone.
This glossary is our attempt to fix that. Every definition is written in plain language — the kind you’d use to explain it to a coworker, not the kind you’d put in an RFC. We’ll keep updating it as the industry invents new ways to say old things.
Bookmark it. You’re going to need it. Consider this your field guide for navigating the wilds of telecom terminology — no need to moose around in confusion.
A
ACD (Automatic Call Distribution)
The system that routes incoming calls to the right person or group. When you call a company and get sent to the next available agent in the support department, that’s ACD doing its job. Most hosted PBX platforms include this as a standard feature.
Analog
The old way of transmitting voice — as a continuous electrical signal over copper wire. This is what traditional landlines use. It works, but it’s limited: one call per line, no data, no flexibility. Most of the world has moved on.
Auto-Attendant
The automated voice menu that answers your calls. “Press 1 for sales, press 2 for support.” Configured properly, it saves your team from answering every call manually. Configured poorly, it makes callers want to throw their phones. There’s a middle ground.
ATA (Analog Telephone Adapter)
A small box that connects a traditional analog phone to a VoIP system. Handy if you have analog phones you like and don’t want to replace them, or if you need to connect a fax machine, elevator phone, or door buzzer to a modern system.
B
Bandwidth
The amount of data your internet connection can carry at once, measured in megabits per second (Mbps). VoIP doesn’t need much — roughly 100 kbps per call — but it needs that bandwidth to be consistent. A 500 Mbps connection that drops packets every few seconds is worse for voice than a stable 50 Mbps one. See also: what actually affects call quality.
BLF (Busy Lamp Field)
Lights on your desk phone that show whether your coworkers are on a call. Green means available, red means busy. Simple and genuinely useful — lets a receptionist see at a glance who can take a transfer without putting the caller on hold to go find out.
C
Call Park
A feature that lets you place a call in a shared “parking spot” so someone else can pick it up from any phone. Common in offices where someone might need to walk to another room or department to handle a call. Think of it as a holding area that anyone can reach.
Caller ID / CNAM
Caller ID shows the phone number of an incoming call. CNAM (Caller ID Name) is the name that shows up with it. For businesses, getting your outbound caller ID right — so customers see your company name, not a random number — matters more than most people realize.
CDR (Call Detail Record)
A log of every call your system handles: who called, who answered, when, how long, what number was dialed. Useful for billing, troubleshooting, and figuring out why Mondays are always chaos. Every decent phone system keeps these.
Codec
Short for coder-decoder. It’s the algorithm that compresses and decompresses your voice for transmission over the network. G.711 is the standard (uncompressed, sounds great, uses more bandwidth). G.729 compresses more aggressively (uses less bandwidth, sounds a little worse). Opus is the newer kid that adapts on the fly. Your provider picks the codec — you mostly don’t need to think about it.
CRM Integration
Connecting your phone system to your customer relationship management software (Salesforce, HubSpot, etc.) so that when a customer calls, their record pops up automatically. Nice when it works. When it doesn’t, it’s usually because someone skipped the setup.
D
DID (Direct Inward Dialing)
A phone number that routes directly to a specific person or extension without going through a main menu. If your employee has “their own number” that rings their desk, that’s a DID. You can have as many as you want — they don’t require individual phone lines.
DNIS (Dialed Number Identification Service)
Tells your phone system which number the caller actually dialed. If your business has multiple phone numbers (one for sales, one for support, one for a specific campaign), DNIS is how the system knows which one was called and routes accordingly.
DTMF (Dual-Tone Multi-Frequency)
The tones your phone makes when you press the number keys. When an auto-attendant says “press 1,” it’s listening for DTMF tones. This is one of those things that just works until it doesn’t, usually because a network is stripping or corrupting the tones.
E
E.164
The international standard format for phone numbers. In the US, that’s +1 followed by the 10-digit number (e.g., +13125551234). Matters because SIP systems use E.164 formatting, and getting the number format wrong is a surprisingly common source of routing problems.
E911 (Enhanced 911)
Emergency calling that includes your location information. With traditional landlines, the phone company knows where you are. With VoIP, you need to register your address so that 911 dispatchers can find you. This is not optional — it’s legally required, and any provider who glosses over it is waving a red flag.
F
Failover
What happens when your primary system goes down. Good failover means calls automatically reroute — to a backup server, to cell phones, to another location — and your callers never know anything happened. Bad failover means silence. This is one of those things you don’t think about until you need it. See our business continuity guide for more.
Firewall
Network security hardware or software that controls which traffic gets in and out of your network. Firewalls are essential, but they’re also the number one cause of VoIP issues in new installations. SIP uses specific ports that need to be open, and if the firewall is blocking them, your calls will fail in creative and frustrating ways.
FXO / FXS
FXO (Foreign Exchange Office) and FXS (Foreign Exchange Station) are port types for connecting analog devices. FXS ports connect to phones. FXO ports connect to phone lines. If you’re integrating analog equipment with a modern system, you’ll encounter these terms. Otherwise, you can safely forget them.
G
Gateway
A device that bridges two different types of phone networks — usually analog/digital and VoIP. If you have old analog phones or a legacy PBX and want to connect them to a VoIP system, a gateway handles the translation.
H
Hosted PBX
A phone system where all the heavy lifting — call routing, voicemail, auto-attendants, everything — runs on your provider’s infrastructure instead of hardware in your office. You just plug in phones and go. It’s what we do. We wrote a whole guide about it.
Hunt Group
A group of phones that ring in a specific pattern when a call comes in. “Ring all at once” (everyone’s phone rings), “round robin” (take turns), or “linear” (try the first person, then the second, then the third). Simple, effective way to make sure calls get answered.
I
IVR (Interactive Voice Response)
The more sophisticated cousin of an auto-attendant. An IVR can look up account information, take payments, route calls based on database lookups, and handle interactions without a human. That system that reads you your bank balance? IVR. For most small businesses, an auto-attendant is plenty. IVR is for when you need the system to actually do things, not just route calls.
IRSF (International Revenue Share Fraud)
A type of phone fraud where attackers hack your phone system and route thousands of calls to premium international numbers they control, racking up charges you’re stuck paying. It’s more common than you’d think. See our guide on PBX hacking for the full story.
J
Jitter
Variation in the delay between voice packets arriving at their destination. Consistent 30ms delay? Fine. Delay bouncing between 10ms and 200ms? That’s jitter, and it makes calls sound choppy and garbled. A jitter buffer in your phone helps smooth this out, but too much jitter and there’s only so much the buffer can do. More in our call quality guide.
L
Latency
The time it takes for your voice to travel from your mouth to the other person’s ear. Under 150 milliseconds is fine. Over 300ms and conversations start to feel like you’re talking to someone on the moon. Latency is mostly a function of physical distance and network routing.
LNP (Local Number Portability)
The legal right to take your phone number with you when you change providers. Porting numbers is straightforward if your provider knows what they’re doing. We’ve ported hundreds of thousands, so we know the gotchas.
M
MOS (Mean Opinion Score)
A numerical score (1 to 5) that rates call quality. 4.0+ is toll quality — indistinguishable from a traditional phone call. 3.5 is acceptable. Below 3.0 and people start complaining. It’s subjective by design (it’s literally derived from people’s opinions), but it’s the standard way the industry benchmarks voice quality.
MOH (Music on Hold)
The music (or messaging) callers hear when they’re on hold or waiting in a queue. Seems trivial, but silence makes callers hang up — they think they’ve been disconnected. Bad music makes them want to hang up. Pick something inoffensive and keep the volume consistent.
N
NAT (Network Address Translation)
A networking function that lets multiple devices on your internal network share a single public IP address. It’s how your office works: 50 computers, one public IP. NAT is fine for web browsing but causes headaches for SIP because it rewrites the IP addresses inside packets. SIP ALG (see below) was invented to “fix” this, and usually makes things worse.
Number Porting
The process of moving your existing phone numbers from one provider to another. It takes anywhere from a few days to a few weeks, depending on the carrier and number type. It should be painless. Often isn’t, unless your new provider has done it thousands of times. We have.
P
PBX (Private Branch Exchange)
The phone system that manages all the internal phone functions for a business — extensions, transfers, voicemail, call routing, all of it. Historically, this was a physical box in a closet. Now it’s usually hosted in the cloud.
Packet Loss
When voice data packets don’t arrive at their destination. At 1%, you won’t notice. At 3-5%, you hear gaps. At higher levels, the call falls apart. Usually caused by network congestion or poor-quality network equipment.
POTS (Plain Old Telephone Service)
Exactly what it sounds like — traditional analog phone service over copper wire. The original. Still works, increasingly expensive, and the phone companies are actively moving away from maintaining the copper network. If you’re still on POTS, it’s probably time.
PRI (Primary Rate Interface)
A digital phone line that carries 23 simultaneous voice calls over a physical T1 circuit. It was the standard for business phone service before SIP trunking came along. Still exists, still works, but increasingly hard to justify the cost when SIP trunks are cheaper and more flexible.
PSTN (Public Switched Telephone Network)
The global network of traditional phone infrastructure — the wires, switches, and exchanges that connect every phone number in the world. When you make a call that goes to a landline or cell phone, your VoIP call eventually touches the PSTN. It’s the common language all phone systems ultimately speak.
Q
QoS (Quality of Service)
Network settings that prioritize voice traffic over other data. When your network is busy, QoS ensures that phone calls get first dibs on bandwidth instead of competing with someone’s file download. Properly configured QoS is the single most impactful thing you can do for call quality on your local network.
R
RTP (Real-time Transport Protocol)
The protocol that actually carries your voice data during a call. If SIP is the handshake that sets up the call, RTP is the conversation itself. It uses UDP (not TCP) because speed matters more than guaranteed delivery for real-time audio — a retransmitted voice packet that arrives late is useless.
Ringback
The ringing sound you hear when you’re waiting for someone to pick up. Fun fact: that sound is generated by the network, not the other person’s phone. It’s telling you “we found them, we’re ringing them, hang on.”
S
SBC (Session Border Controller)
A network device that sits at the edge of a VoIP network and handles security, NAT traversal, and protocol translation. Think of it as a bouncer and translator for your voice traffic. Enterprise environments and service providers use them heavily. Most small businesses never interact with one directly.
SIP (Session Initiation Protocol)
The signaling protocol that sets up, manages, and tears down VoIP calls. SIP handles the “hello, I’d like to make a call to this number” part. It’s the standard that makes different phone systems talk to each other. When someone says “SIP phone” or “SIP trunk,” they’re talking about devices or services that use this protocol.
SIP ALG (Application Layer Gateway)
A router feature that tries to “help” SIP traffic pass through NAT by rewriting SIP packets. In theory, useful. In practice, it breaks things more often than it fixes them. The first troubleshooting step for almost any VoIP issue behind a router: turn off SIP ALG.
SIP Trunk
A virtual phone line that connects your PBX to the phone network over the internet using SIP. Replaces physical phone lines (PRI, POTS). You buy as many simultaneous call paths as you need, often for significantly less than traditional lines. It’s how most modern business phone systems connect to the outside world.
Softphone
A software application that turns your computer or smartphone into a phone. Same functionality as a desk phone — dial, answer, transfer, hold — but running on a device you already have. Great for remote workers or anyone who doesn’t want another piece of hardware on their desk.
SRTP (Secure Real-time Transport Protocol)
The encrypted version of RTP. Scrambles your voice data so that anyone intercepting it hears noise instead of your conversation. If call privacy matters to you, ask your provider whether they support SRTP. If they look confused, find a different provider.
STIR/SHAKEN
An industry framework for authenticating caller ID to combat robocall spoofing. The calling provider cryptographically signs the call, and the receiving provider verifies it. It doesn’t block spam calls directly, but it makes it much harder to fake the number they’re calling from. Required by the FCC for US carriers.
T
T.38
A protocol for sending faxes over IP networks. Regular voice codecs mangle fax tones, so T.38 was created to handle the fax data separately. If you still send faxes (and some industries have to), make sure your provider supports T.38 — otherwise your faxes will fail in unpredictable ways.
TLS (Transport Layer Security)
Encryption for SIP signaling. If SRTP encrypts the voice (the conversation), TLS encrypts the setup (who’s calling whom, what number was dialed). Together, TLS + SRTP means your entire call is encrypted end to end.
Trunk
In telecom, a trunk is a shared communication line that carries multiple calls between two points. A SIP trunk carries calls over the internet. A PRI trunk carries calls over a T1 circuit. The concept is the same: a pipe that multiple calls share, rather than needing a dedicated wire for each one.
U
UCaaS (Unified Communications as a Service)
A cloud-delivered package that bundles voice, video, messaging, and collaboration tools into one platform. It’s the industry’s current favorite buzzword. Some businesses genuinely need all those tools integrated. Others just need phones that work and end up paying for video conferencing they never use. Know what you actually need before you buy the bundle.
UDP (User Datagram Protocol)
A network protocol that sends data without waiting for confirmation that it arrived. Faster than TCP but less reliable. Voice traffic uses UDP because speed trumps reliability — you’d rather lose a tiny fragment of audio than have the whole call pause while the network retransmits it.
V
Voicemail to Email
Exactly what it says — your voicemail messages get delivered to your email inbox, usually as an audio file attachment and sometimes with a text transcription. Simple feature, surprisingly life-changing for people who never check their voicemail the traditional way.
VoIP (Voice over Internet Protocol)
The technology that transmits voice calls over the internet instead of traditional phone lines. It converts your voice into data packets, sends them across the network, and reassembles them on the other end. It’s how most phone calls work today, whether you realize it or not. If you’re reading this glossary, you probably already know this one — but here’s the full picture of what it costs.
W
WebRTC (Web Real-Time Communication)
A browser-based technology that enables voice and video calls directly in a web browser without installing any software or plugins. It’s what powers most click-to-call buttons on websites and browser-based softphones. Useful for lightweight communications where a full softphone app is overkill.
Still Have Questions?
Telecom jargon exists for a reason — these terms describe real, distinct things. But nobody should need a decoder ring to understand their phone system.
If you’ve read through this and still have questions, or if you’ve run into a term we didn’t cover, drop us a line. We’re happy to explain anything in plain language — no 47-slide deck required. And if you’re evaluating a phone system and want someone to translate what other providers are telling you, we’ll do that too. Even if you don’t end up choosing us.