The primary purpose of registration is for the Digium device to know where to send calls toward the registered endpoint. It is a mapping in the Digium that says SIP-identity XXX is at IP address YYY. Secondarily, the authentication associated with registration is used to verify that the authenticated endpoint is permitted to place calls. Registration doesn't tell the Adtran what calls the Digium is expecting to handle.
The TA924e needs to have a voice SIP trunk pointing to the Digium in order to route calls to it, as well as a grouped-trunk specifying which calls.
Just because you have an FXS port registered to the Digium doesn't necessarily mean that you want to route all outbound calls there. The TA924e is flexible enough to allow you to create voice routing to multiple trunks based on digit patterns. In your case, a straightforward voice trunk pointing to the Digium with a grouped-trunk default will work just fine. If you have other voice trunks you can (and may need to) get much more granular.
An alternative way to do this is to have the Adtran trunk register to the Digium rather than the FXS endpoint(s) but this is likely more complex than you need.