Blog / IT Terms Explained

What is VoIP, and what does SIP have to do with it?

· 2 min read · By the Comdeal team

In short

VoIP means making phone calls over a data network. SIP is the language that sets those calls up. Here is how the two fit together.

VoIP in one sentence

VoIP stands for Voice over IP. It means your voice is turned into small data packets and sent over a network, instead of travelling down a dedicated copper phone line. If you have ever made a call through Teams or WhatsApp, you have used a form of it.

So where does SIP come in?

SIP, the Session Initiation Protocol, is the signalling language of most business phone systems. It starts a call, changes it and ends it. Think of it as the short conversation that happens before you can talk: is that number reachable, which voice format should we use, and where should the audio go?

The voice itself travels separately, usually as a stream called RTP. That split matters when something goes wrong: signalling can work perfectly while the audio is silent or choppy.

The parts you will hear about

  • IP phone or softphone: the handset or app that makes and receives calls.
  • PBX: the phone system that connects your extensions to each other and to the outside world. It can sit in your office or be hosted.
  • SIP trunk: a virtual phone line between your PBX and a provider, replacing old ISDN or analogue lines. Capacity is counted in simultaneous calls, called channels.
  • Codec: how the voice is compressed. G.711 sounds great and uses more bandwidth, G.729 uses much less at a small cost in quality.
  • SBC: a session border controller. It sits at the edge of the network to secure SIP and make different systems work together, for example when connecting a phone system to Microsoft Teams.

Why businesses move to it

Numbers can follow people between sites, adding a line is a configuration change rather than a technician visit, and call costs and line rental often drop. The catch is that voice is fussy about network quality. Delay, jitter (uneven timing) and packet loss all show up as bad audio, so the network under your phones matters as much as the phones.

Questions worth asking any provider

  1. How many simultaneous calls will I need, and what happens when they are all busy?
  2. What happens to calls if my internet connection fails?
  3. Who looks after the network side as well as the phone side?
  4. Can I keep my existing numbers?
Not sure how much bandwidth your calls need? Try the VoIP bandwidth calculator, or the Erlang calculator to size your trunk.

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