Cisco Callmanager to Asterisk (ViaTalk) SIP Configuration

I am trying to connect to ViaTalks Asterisk proxy from our Cisco CallManager servers. Since ViaTalk requires registration with a userid (phone number)/password combination I am forced to setup the SIP connection on a 3640 router rather than on the callmanager servers since callmanager does not support sip trunk registrations. ViaTalk has a configuration example for CallManager Express but I can’t seem to get it to work with plain callmanager. Has anyone setup a SIP Registration from Callmanager/Router to ViaTalk or another VOIP provider that has a configuration they can share or any ideas?