Did you ever get this sorted? I am having a very similar issue at the moment.
MBG is server gateway, no firewall in place. MBG is registering with provider, and everything appears to be working correctly. However there is a timeout between the MBG and the MCD. MCD keeps dropping the SIP link due to an options timeout, but a packet trace shows MBG requesting and receiving options from the provider. Incoming call is picked up by the MBG, but does appear to pass onto the MCD.