Systems and methods can be implemented for IP multimedia subsystem (IMS) resource reservation. A first electronic device can transmit, through an IP network, a non-Session Initiation Protocol (SIP) message to an application server in an IMS network, where the non-IMS SIP message includes a request to initiate an IMS communication session through the IMS network with a second electronic device. A SIP message can also be transmitted through the IMS network to the application server that initiates a SIP INVITE session between the first electronic device and the application server, where the SIP INVITE session is routed through the IMS network. The first electronic device can then, receive a SIP response message that identifies a resource reservation for the IMS communication session from the application server.