I have a CME that I am using as a centralized call manager for the CME site and 2 remote sites. The 2 remote sites have MPLS circuits, but to setup SRST the local telco services in the area are low quality and unusable. A plan was to setup SIP trunking on the CME with DID numbers for the area that doesn't have usable local services and calls to these numbers would be routed to that site so that it would appear that it was a local service. The SIP trunking would also provide outbound calling from the area with the unusable local services. For a SIP call to tranverse between the 2 locations is CUBE required on the remote routers for IP to IP calls. All phones register to the CME and calls presently are all sent to and from the CME PSTN connections with the idea of the SIP trunking just being another PSTN connection.
I know this is a very old thread but just for future references you need to be more specific regarding the details of your question like why do you want to enable SIP trunking between the branches routers if you already have the phones registered at the main site (CME site) CME also why do you need the SRST if your problem is with the Telco service provider (are you referring to the WAN service provider in your case) ?