Voice over internet protocol multimedia subsystem

A non-transitory computer readable memory medium storing program instructions executable by processing circuitry to cause a user equipment device (UE) to perform operations comprising:

• sending, to a communications network associated with a cell of a radio access technology (RAT) on which the UE is camped, an IMS session initiation protocol (SIP) registration message;
• receiving, from the communications network, a message indicative of a SIP rejection;
• determining, based on the message indicative of the SIP rejection, that the SIP rejection is temporary;
• starting an IMS retry timer, wherein the UE is configured to reattempt SIP registration upon expiration of the IMS retry timer;
• and
• sending to the communications network an IMS failure message that comprises at least one of an IMS failure cause or an IMS retry timer.

A method to be performed by a user equipment (UE), the method comprising:

• sending, to a communications network associated with a radio access technology (RAT) on which the UE is camped, an IMS session initiation protocol (SIP) registration message;
• receiving, from the communications network, a message indicative of a SIP rejection;
• determining, based on the message indicative of the SIP rejection, that the SIP rejection is temporary;
• starting an IMS retry timer, wherein the UE is configured to reattempt SIP registration upon expiration of the IMS retry timer;
• and
• sending to the communications network an IMS failure message that comprises at least one of an IMS failure cause or an IMS retry timer.