asterisk users - Jan 2006 - Force Port Number on INVITE

Hi

My VoIP provider demands that the address on the INVITE line includes
the port number, so:

INVITE sip:12345@1.2.3.4:5060 SIP/2.0

Not:

INVITE sip:12345@1.2.3.4 SIP/2.0

I can see example logs from other users where this appears to have been
done, but I can't find how to do it.  Any suggestions?

I apologise if the answer is obvious!

David