Embodiments are directed to selecting a physical layer for an access terminal's (AT's) participation in a communication session. In an embodiment, the AT can register its priorities for multiple physical-layer systems as well as contact information by which an application server can contact the AT over each system. The AT selectively updates the system prioritization and/or contact information. When the AT joins or initiates a communication session, the application server supports the AT on a highest-priority system through which the AT can be contacted. The system supporting the AT's session can change upon request by the AT, the initiative of the application server and/or a detected triggering event. In a further embodiment, multiple systems can be used concurrently to support the AT's session, such that the AT can send and/or receive signaling and/or media for at least a portion of the communication session over the multiple systems concurrently.