SIP Trunk config: non-static-IP trunks

Overview: this article is aimed at our installers and client IT staff who may wish to configure their own PBX equipment. The model used in this example is Grandstream, but the principles and settings are similar on most other models such as 3CX, Sangoma, and other systems based upon Asterisk.

You will need:

• Admin access to the PBX
• SIP username & password
• Optional: SIP TLS certificate, if you want to use SRTP
• Number ranges: list of valid number ranges

Concept

The PBX will initiate a connection to the SIP server via a proxy server. Since a static IP address is not used to validate, your PBX will need to register using a SIP username and password. If authentication succeeds, 2-way SIP traffic will be allowed providing that (1) in the case of inbound calls a user matching the phone number exists on your PBX, and (2) for outbound call the user's phone number is both valid and  presented in UK international format [44 1234 567 890] beginning 44.

Example using Grandstream PBX

• Create SIP trunk
• Set inbound and outbound routing rule so that calls use the SIP trunk

Step 1: add trunk basic settings

Create a new trunk, for provider name you could call it "vodafone". The Host Name (aka. "domain") is "entvoice.vodafone.co.uk", the username and password are the SIP username/password that you will have been provided with. We also need to configure the "advanced" tab:

Step 2: add trunk advanced settings

On the advanced tab enable "PAI Header" (P-Asserted-Identity. which verifies that a user is valid for any calls). Now enable "Outbound proxy", and enter "10b.Z3.rtp.entvoice.vodafone.co.uk" as the proxy and set SRTP to "disabled"; we do supply a TLS certificate if you want to use SRTP - to use SRTP change the proxy to "10b.Z3.srtp.entvoice.vodafone.co.uk" and install the supplied TLS certificate.

Step 3: Save and test

Once the new SIP trunk has been saved we recommend making test calls in and out; before doing so, please ensure that the PBX has a default outbound call route that matches the new SIP trunk, and that the user account on the PBX has a valid phone number (for use with this trunk).

Tip: ensure that your inbound routing rule caters for all possible variants of the number format:- local, international, and international with plus symbol. Example shown here:

For further assistance we invite you to open a ticket detailing the PBX make & model, along with a description of the assistance needed.