Can you provide a scrubbed (No passwords or public IP information) version of the two configs? I would guess if your never seeing the registration sip message hitting the 7100 that the SBC either can't access the 7100, or one of them is not setup with the proper trunking config. If the SBC can ping the 7100 then I would make sure that the interfaces that the 7100 and SBC talk to each other on have the following configured on the interfaces:
media-gateway ip primary
Also make sure on the 7100 to set the user accounts connecting from the SBC as Remote Phones. Other things to check is make sure you have the VLAN routing setup so the VLAN the SBC connects from can talk to the voice VLAN you have configured on the 7100. You can test this by pinging the Voice VLAN IP address from the SBC.
To provide much more I would need to see the configs so I can get a better idea of how everything talks. Also is there a special reason you are having the remote phone connect to the SBC instead of just allowing to connect directly to the 7100.
Have you verified connectivity? Can you ping between the two devices? Any ACLs or firewall that would cause an issue? Is the interface towards the other device configured "media-gateway ip primary" on both sides?
Thanks for the info guys. Issue resolved. Apparently the connectivity was not working. Cabling issue.