When a 8800 series phone is registered over MRA, where does it register?

Even when connecting via Mobile and Remote Access (MRA), the Cisco 8800 series phone still registers with the Cisco Unified Communications Manager (CUCM) cluster within the corporate network. Cisco Expressway-E and Expressway-C facilitate the secure traversal of registration and signaling traffic through firewalls and NAT, but CUCM remains the core call processing and registration server.

Expressway-C (internal) is a component of the MRA solution that relays registration requests and signaling between Expressway-E and CUCM, but the phone itself does not register to Expressway-C; it registers through it to CUCM.

Cisco Unified Presence Server (now Cisco Unified IM and Presence) handles instant messaging and presence services, not phone registration for call control.

Expressway-E (external) is the internet-facing component of the MRA solution that handles connections from outside the network. Similar to Expressway-C, the phone connects to Expressway-E, but its registration is then proxied through Expressway-E and Expressway-C to CUCM.