Help with multiple CID on a single SIP trunk

Howdy community,

I’m looking for some assistance with a issue that I have not been able to call out when the CID is outside the nominal standard extension used by the ITSP

FreePBX 2.11.0
Asterisk 11.6.0

After discussions with our ITSP, we have moved from hundreds of SIP trunks, one per extension, to just 1 trunk with multiple CIDs outgoing. Add to this, we have a ISDN number range from another Telco, and the ITSP has agreed to allow this as the we can provide evidence of ownership.

Our problem is that if we send a From: that is from the ISDN number range, the call is rejected by the ITSP as unauthorised.

Discussions with the ITSP have indicated that we dont have the correct header information.

We must have a static header containing the Primary account number (in this case the Trunk authorised username) in the Contact: field.

From: sip:[email protected];tag=as7b65a2e1
To: sip:[email protected]
Contact: sip:[email protected]:5060

So the From and To fields are dynamic, but the Contact field is static.

Makes no difference with the Sip settings NAT configuration.

Any help would be appreciated.

The example SDP didnt post. Here is it again.

From: ;tag=as7b65a2e1 To: Contact: