VoIP Glossary: 40 Terms Every Business Owner Should Know

VoIP comes with its own language. Sales reps, providers, and IT teams throw around acronyms and jargon that can make choosing and managing a phone system feel unnecessarily complicated.

This glossary cuts through the noise. It covers 40 terms you will actually encounter when researching, buying, or using a business VoIP system — explained in plain English without the technical waffle.

Core VoIP Terms

1. VoIP (Voice over Internet Protocol)

Technology that sends voice calls over the internet instead of traditional phone lines. Your voice is converted into data packets, transmitted across your internet connection, and reassembled as audio at the other end.

2. SIP (Session Initiation Protocol)

The signalling protocol that sets up, manages, and ends VoIP calls. When you dial a number, SIP handles the "handshake" between your phone and the server. It does not carry the actual audio — that is RTP's job.

3. RTP (Real-time Transport Protocol)

The protocol that carries the actual voice audio during a call. RTP works alongside SIP: SIP sets up the call, RTP delivers the sound.

4. SIP Trunk

A virtual phone line that connects your phone system (PBX) to the public phone network via the internet. Instead of physical copper lines, SIP trunks use your internet connection. You can have as many simultaneous calls as your bandwidth allows.

5. Hosted VoIP

A VoIP service where the provider manages all the phone system infrastructure in the cloud. You just plug in phones and make calls. No on-site PBX hardware required. For a full comparison of hosted VoIP options, see our hosted VoIP solutions guide.

6. On-Premises PBX

A phone system where the central hardware sits physically in your office. You own and maintain it. Offers maximum control but requires upfront investment and ongoing IT management.

7. Cloud PBX / Hosted PBX

A PBX system hosted in the provider's data centre rather than in your office. You access it over the internet. Functionally identical to hosted VoIP for most business purposes.

8. UCaaS (Unified Communications as a Service)

A cloud platform that bundles voice calls, video conferencing, messaging, file sharing, and presence into a single service. Microsoft Teams, Zoom, and RingCentral are examples. VoIP is one component of UCaaS.

Call Quality and Network Terms

9. Codec

Software that compresses and decompresses voice audio for transmission. Common codecs include G.711 (uncompressed, best quality), G.729 (compressed, saves bandwidth), and Opus (modern, adaptive). Your codec choice affects both audio quality and bandwidth usage.

10. Latency

The time delay between speaking and the other person hearing you, measured in milliseconds. Below 150ms is acceptable for voice; below 80ms feels natural.

11. Jitter

Variation in packet arrival times. Packets should arrive at regular intervals. High jitter means some arrive late and get discarded, causing choppy audio. Below 30ms is acceptable; below 15ms is ideal.

12. Packet Loss

The percentage of data packets that fail to reach their destination. Even 1-2% packet loss makes VoIP calls sound choppy. Below 0.5% is the target.

13. MOS (Mean Opinion Score)

A standardised rating of voice call quality on a scale of 1 to 5. A MOS of 4.0+ is considered good quality. Below 3.5 indicates noticeable degradation. Your VoIP provider should be able to show MOS scores for your calls.

14. QoS (Quality of Service)

Router and network settings that prioritise voice traffic over other data. Without QoS, a large file download can degrade your calls. With it, voice packets always get priority.

15. Bandwidth

The maximum data transfer capacity of your internet connection, measured in Mbps. Each VoIP call uses approximately 100 Kbps with the G.711 codec. Bandwidth is necessary but not sufficient — a high-bandwidth connection with poor stability still delivers bad calls.

Phone System Features

16. Auto Attendant / IVR (Interactive Voice Response)

An automated system that answers calls and routes them using voice prompts or keypad input. "Press 1 for Sales, Press 2 for Support" is a basic IVR. Modern systems can use speech recognition and AI.

17. Call Queue

A feature that holds incoming calls in line when all agents or extensions are busy. Callers hear hold music or messages while waiting for the next available person.

18. Ring Group

A group of extensions that ring simultaneously or in sequence when a specific number is called. Useful for departments — all sales phones ring when someone calls the sales number.

19. Hunt Group

Similar to a ring group but with specific routing logic — calls can be distributed round-robin, to the least busy extension, or in a fixed sequence.

20. Call Forwarding

Redirecting calls from one number to another. Can be unconditional (always forward), conditional (forward when busy or unanswered), or time-based (forward after hours to a mobile).

21. Call Recording

Automatically recording calls for training, compliance, or dispute resolution. UK businesses must inform callers when recording. GDPR applies to stored recordings.

22. Voicemail to Email

Voicemail messages converted to audio files (usually MP3 or WAV) and delivered to your email inbox. Many modern systems also include voicemail transcription — converting the audio to text.

23. Hot Desking

The ability for any user to log into any phone and have their extension, settings, and voicemail follow them. Essential for flexible working and shared desk environments.

24. Softphone

A software application that turns a computer, tablet, or smartphone into a VoIP phone. No physical handset needed. Popular options include Zoiper, MicroSIP, and provider-branded apps.

25. WebRTC (Web Real-Time Communication)

Technology that enables voice and video calls directly in a web browser without plugins or software installation. Many modern VoIP platforms use WebRTC for their browser-based calling features.

Number and Line Terms

26. DDI (Direct Dial-In)

A phone number that routes directly to a specific extension or user without going through a receptionist or auto attendant. Each employee can have their own DDI number.

27. CLI (Calling Line Identification)

The phone number displayed to the person you are calling. Also known as Caller ID. In VoIP, you can typically choose which number to display for outbound calls.

28. Geographic Number

A phone number tied to a specific area code (e.g., 020 for London, 0161 for Manchester). Gives callers a local presence and is generally trusted more than non-geographic numbers.

29. Non-Geographic Number

A number not tied to any location (e.g., 03 numbers). Charged at the same rate as local calls. Useful for businesses that operate nationally and do not want to appear tied to one area.

30. Number Porting

Transferring your existing phone numbers from one provider to another. You keep your numbers when switching VoIP providers. The process typically takes 1-10 working days in the UK.

Infrastructure and Security Terms

31. NAT (Network Address Translation)

The process your router uses to let multiple devices share a single public IP address. VoIP is sensitive to NAT because call protocols embed IP addresses in their packets. Misconfigured NAT is a major source of VoIP issues.

32. SIP ALG (Application Layer Gateway)

A router feature that tries to help VoIP traffic pass through NAT by modifying SIP packets. In practice, it usually causes more problems than it solves. Disabling SIP ALG is one of the first troubleshooting steps for most VoIP issues.

33. VLAN (Virtual Local Area Network)

A way to segment your physical network into separate logical networks. Best practice is to put VoIP phones on a separate VLAN from computers and other devices. This improves call quality and security.

34. PoE (Power over Ethernet)

Technology that delivers electrical power through Ethernet cables, eliminating the need for separate power adapters for VoIP phones. Requires a PoE-capable switch.

35. SRTP (Secure Real-time Transport Protocol)

An encrypted version of RTP that protects voice audio from eavesdropping. Essential for businesses handling sensitive information. Ask your provider if they support SRTP.

36. TLS (Transport Layer Security)

Encryption for SIP signalling traffic. While SRTP encrypts the audio, TLS encrypts the call setup information — who called whom, when, and for how long.

37. STUN (Session Traversal Utilities for NAT)

A protocol that helps VoIP devices discover their public IP address when behind NAT. Works with most NAT types and is essential for correct audio path establishment.

Business and Commercial Terms

38. SLA (Service Level Agreement)

A contractual guarantee from your provider covering uptime (typically 99.9% or 99.99%), response times for support, and compensation if they fail to meet targets. Always check the SLA before signing.

39. Per-User / Per-Seat Pricing

The most common VoIP pricing model. You pay a monthly fee for each user or extension. Typically ranges from £6 to £25 per user per month in the UK, depending on features. For detailed pricing, see our small business VoIP comparison.

40. Concurrent Call / Channel

The number of calls that can happen simultaneously on your system. With SIP trunking, you purchase a number of channels rather than physical lines. Each channel supports one concurrent call.

Putting It All Together

You do not need to memorise all of these terms. But knowing the basics — especially SIP, RTP, codec, QoS, and SIP trunk — will help you have more informed conversations with VoIP providers and make better decisions about your phone system.

When you are evaluating providers, use this glossary to decode their sales materials. If a provider cannot explain their service in terms you understand, that tells you something about how they will handle support when you need it.