I have a CUCME v9 setup in my home lab whilst I train for CCNP Voice. Recently I've started playing with SIP ITSP's and I currently have two separate SIP trunks configured on this device. One to CallCentric and one to an Australian company called Faktortel which I use for local calls back to my home in England, which is where the CUCME is located.
I have everything working fine but I noticed that incoming calls are never to my DID number which I have, from either ITSP. The "incoming called number" is always my SIP username. For logical reasons on my part, I use translation rules to change this back to my DID, which is then matched to a hunt pilot which in turn rings a bunch of phones in parallel.
To investigate this, I captured some SIP INVITE messages from my SIP ITSPs using debug ccsip message. I notice they are both SIPv2 but they look different.
I was thinking rather if I were to purchase multiple DID numbers, how the system would know how to route those based on the SIP messages.
Thanks for the info and I'm aware its an open protocol. Although my system must know what to do with the messages to be able to act on them. For example, I found it strange that one message says "From" and the other says "f". There must be a base standard to work from.