SIP trunking Authentication with seperate username and registration address (AOR)

I am running into an issue authorizing invites from my asterisk box connecting to my VoIP provider via a SIP trunk. They supplied me with a username and password to authenticate and then a registration address or the address of record. The address of record or registration address is needs to be in the FROM field on the invite, and the auth name needs to be displayed in the DIGEST portion of the auth invite.

Currently the trunk will register with my providers SBC because it registers based on AOR not the digest username. This wont allow my trunk to make or receive calls because of the authentication issues.

I need to know how its possible to have a seperate AOR which is sometimes referred to with FreePBX as the DID and then a seperate username.

Any and all help are greatly appreciated.