SIP trunk

A SIP trunk is a method for sending voice and video calls and other types of communication  – across the Internet – between two endpoints.

In this article we will explain what a SIP trunk is and how you can get one for your business.


What are SIP trunks for?

A SIP trunk allows you to connect some telephony hardware or software on your side – across the public Internet – to the regular telephone network on the other side. This allows you to receive calls from the telephone network – to some phone number you have – and to make regular calls to mobiles and landlines.

SIP trunks are useful if you need to make or receive many calls at the same time. This can be the case if you have a Call center or a business with multiple employees behind a company switch (PBX).


What is SIP?

SIP stands for “Session Initiation Protocol” and is a standard that allows two endpoints (phones, switches etc.) to establish contact with each other across the Internet and set up a session, such as a voice call between two persons.

The actual audio or video in calls doesn’t travel across the SIP connection. SIP is only used for agreeing on what to send (voice, video etc.) in what format (some codec for sending voice or video as packets across the Internet) and whereto (some IP address and port).

Think about it as a dialogue.

“Hi there stranger, I have a voice call for you”

“Howdy! OK. I’m happy to receive a voice call from you. Here’s a list of voice formats I am able to handle.”

“Aha. OK. The last format in your list is the only one I can handle myself. Let’s use that.”

“Sure thing. I’ll start ringing the phone on my side then. “

“OK. Sounds good. I’ll play ringing tone to the caller on my side.”

“Alright. Someone picked up the phone on my side now. Please start sending your call audio to this IP-address.”

“Great! Will do. And please send your audio to me at this IP-address.”

“Okidoki. Done. The audio call is set up both ways now. Nothing more for us to do right now. Let me know if something happens.”

“Sure thing. Will do!”

“Oups, My side hung up. Disconnecting audio from my side. Bye now”

“Alright. Stopping the audio from my side as well. Bye for now”


What does “trunk” mean?

The term “trunk” is borrowed from the old traditional telephone network, where it generally referred to a physical cable with the capacity of handling a certain amount of simultaneous phone calls.

A trunk could for example handle 30 or 24 simultaneous calls and could connect a company phone switch to the regular telephone network.

“Trunk” in the context of “SIP trunks” refers to the connection between your equipment/software and the equipment/software of some service provider – and the max capacity of simultaneous calls that the service provider allows you to have running at any point in time.


How can I get one?

SIP trunks are useful for Call Centers and larger companies with larger call volumes.

Sonetel offers SIP-trunks. You can read more about it at our special site for SIP trunking.