MESSAGE-BASED NOTIFICATION THAT A CALLED PARTY IS BUSY

A VoLTE (Voice over Long-Term Evolution) network comprising 4G (4th generation) LTE and IMS (Internet Protocol Multimedia Subsystem) networks includes an application server configured to provide message-based notifications to user equipment of a calling party when a previously busy called party is free to accept a new voice call. In an illustrative embodiment, a CCBS (Completion of Communications to Busy Subscriber) service signalling flow described by 3GPP (3rd Generation Partnership Project) technical specification TS 24.642 is modified to replace an automated CCBS recall (i.e., call back) to the calling party with a notification comprising an SMS (Short Message Service) message. The SMS notification is sent using Session Initiation Protocol (SIP) proxy servers implementing call session control functions (CSCF) in an IMS core that interoperate with an IP Short-Message-Gateway (IP-SM-GW) between the LTE and IMS networks that appears to the IMS core as another application server.