I'm trying to understand how SIP works and the industry in general. We are planning to setup a PBX (open source) and have SIP phones for our customer care agents.

When I was researching on SIP trunks, I found that some providers give you a range of numbers with the trunk. This is confusing to me, since I imagined that a SIP trunk was linked to a single phone number on the PSTN and we would manage the SIP numbers internally in the PBX.

Could someone please explain the range of numbers and tell me if I have not understood SIP trunks correctly?

1 Answer 1


I'm pretty sure this kind of question is off topic for this site since it deals with application layer protocols, I think.

However, the answer is very simple. Each SIP service provider will have its own methods for providing their various services but yes, typically a SIP trunk (also called SIP call termination and origination) provider will also lease you Direct Inbound Dial (DID) numbers for use on the service.

Each provider will have its own methods for configuring how those numbers are used with the service. When using your own PBX system (as opposed to a PBX service from the provider), the SIP trunk will simply dump all inbound calls to the PBX and allow the PBX system to manage delivery of the calls for any associated DIDs, etc.

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