A method of setting up a communication session in a wireless communications system, comprising: determining, at an access terminal, to initiate a communication session to be arbitrated by an application server; determining a system prioritization for a plurality of systems by which the access terminal can contact the application server, each of the plurality of systems associated with a different physical-layer; sending a registration message to indicate the system prioritization to the application server, wherein the registration message includes contact information that permits data sent over a system interface corresponding to one of the plurality of systems to be recognized by a corresponding system at a radio access network, RAN, and permits a corresponding system at the RAN to send data to the corresponding system interface at the access terminal; attempting to send a session initiation request to the application server over a highest-priority system based on the system prioritization; if the attempt is unsuccessful, re-sending the session initiation request on at least one of the plurality of systems other than the highest-priority system, wherein the re-sending occurs after a threshold number of unsuccessful attempts on the highest-priority system; and if one of the attempts to send the session initiation request is successful, selectively updating the system prioritization with the application server such that the system upon which the successful attempt is made corresponds to the highest-priority system in the updated system prioritization and sending a supplemental registration message to indicate the updated system prioritization to the application server, wherein a determination to selectively update the system prioritization is performed by the access terminal in response to a triggering event related to at least one of the plurality of systems. determining, at an access terminal, to initiate a communication session to be arbitrated by an application server; determining a system prioritization for a plurality of systems by which the access terminal can contact the application server, each of the plurality of systems associated with a different physical-layer; sending a registration message to indicate the system prioritization to the application server, wherein the registration message includes contact information that permits data sent over a system interface corresponding to one of the plurality of systems to be recognized by a corresponding system at a radio access network, RAN, and permits a corresponding system at the RAN to send data to the corresponding system interface at the access terminal; attempting to send a session initiation request to the application server over a highest-priority system based on the system prioritization; if the attempt is unsuccessful, re-sending the session initiation request on at least one of the plurality of systems other than the highest-priority system, wherein the re-sending occurs after a threshold number of unsuccessful attempts on the highest-priority system; and if one of the attempts to send the session initiation request is successful, selectively updating the system prioritization with the application server such that the system upon which the successful attempt is made corresponds to the highest-priority system in the updated system prioritization and sending a supplemental registration message to indicate the updated system prioritization to the application server, wherein a determination to selectively update the system prioritization is performed by the access terminal in response to a triggering event related to at least one of the plurality of systems.