This question is more educational for me, but i have also seen problem related to this scenario....
First of all, call flow is
SCCP Phone --- CUCM --- SIP --- CUBE --- SBC(provider)
At times, i see the INVITE coming from CUCM, and the sequence of establishing the call completes.
At other times, i never see the communication with CUCM, its like the gateway is placing and termiting the call leg and never sending or receiving SIP messages from CUCM. I am guessing that this may have something to do with MTPs, but would like clarification on why this happens, why its intermittent, and how this works.
There are times when i have seen call fail when there was no SIP communication. Typically, the INVITE will go out from CUBE (Again nothing is seen coming from CUCM), the provider responds back, but the result is one way audio, or no audio. This is what leads me to believe its MTP related.
I understand SIP fairly well, but its the CUCM MTP portion, if this is the cause, that confuses me.