What is SIP trunking?
SIP trunking is a service that connects a company's own phone system, such as an on-premise or IP PBX, to the public telephone network over the internet. It replaces physical phone lines with virtual ones that use the SIP protocol.
How it works
SIP (Session Initiation Protocol) is the signaling standard VoIP uses to start and end calls. A SIP trunk is an account with a provider that accepts SIP calls from your PBX and delivers them to the phone network, and the reverse.
Each trunk supports a number of simultaneous calls, called channels. Numbers, often DIDs, are attached to the trunk so that incoming calls reach the PBX and are routed to extensions.
You keep and manage the PBX yourself; the trunk only provides connectivity and numbers. That is the main difference with a cloud phone system, where the provider runs the PBX too.
Why it matters
01Lower line costs
Virtual channels are cheaper and more flexible than analog lines or ISDN circuits.
02Keep your PBX
A company with an existing IP PBX can modernize connectivity without replacing it.
03Scale channels
Add capacity for more simultaneous calls without new wiring.
Allo and SIP trunking
Allo is not a SIP trunk provider. It is a hosted phone system, so there is no PBX to run and no trunk to configure: numbers and calling come with the app.
Related terms
The terms that usually come up in the same conversation.
All glossary terms
Frequently asked questions
VoIP is the general technology for calls over the internet. SIP trunking is a specific VoIP service that connects your own PBX to the phone network.
As many as the calls you expect at the same time at peak, plus a margin. It is not one per employee, since not everyone is on the phone at once.
No. A cloud phone system already includes the connection to the phone network, so there is no separate trunk to buy.