Per https://pubs.vocera.com/edge/server/4.11.0/help/server_edge_emug_help/index.html#edge/server/4.11.0/topics/em_care_role_forwarding.html, the Device Phone is used first with CRF to Cellular, and if not present the call will forward to the callee's Mobile Phone number instead. URMC requested for the user's preferred contact, if set, to be used for CRF to Cellular.

See Salesforce case 00491226 for a scenario prompting this request.