When an 8800 phone is registered over Mobile and Remote Access, where does it register?

Cisco Unified Presence Server (CUPS) handles instant messaging and presence, not IP phone registration for call control.

Expressway-E (external) is part of the MRA architecture, handling external connectivity, but it proxies the registration request to Expressway-C, which then proxies it to CUCM; it is not the registration endpoint.

Regardless of whether a phone is local or remote via Mobile and Remote Access (MRA), the ultimate call control and registration target for Cisco IP phones (like the 8800 series) is always Cisco Unified Communications Manager (CUCM). The Expressway-C and Expressway-E merely proxy the signaling.

Expressway-C (internal) is also part of the MRA architecture, forwarding registration requests from Expressway-E to CUCM, but it is not the entity that actually registers the phone.