A method comprising: receiving, by an application server from a first electronic device, a non-"Internet Protocol Multimedia Subsystem Session Initiation Protocol", non-IMS SIP, message (220, 320) through an IP network, wherein the non-IMS SIP message (220, 320) includes: a request to initiate an IMS communication session through an IMS network between the first electronic device and a second electronic device via the application server; authentication information associated with the first electronic device for validating the first electronic device at the application server, a destination address associated with the second electronic device, and an indication that indicates an association between the non-IMS SIP message (220, 320) and a corresponding SIP message (222, 344a) for initiating the IMS communication session between the first electronic device and the second electronic device via the application server; authenticating, by the application server, the first electronic device based, at least in part, on the authentication information in the received non-IMS SIP message (220, 320); initiating, by the application server, the IMS communication session with the second electronic device in response to the request included in the received non-IMS SIP message (220, 320); receiving, by the application server, the SIP message (222, 344a) that initiates a SIP INVITE session between the first electronic device and the application server, wherein the SIP INVITE session is routed through the IMS network corresponding to the received non-IMS SIP message (220, 320) through an IMS network (204, 206); associating, by the application server, the SIP message (222, 344a) received through the IMS network (204, 206) with the initiated IMS communication session; and sending, by the application server to the first electronic device, a SIP response message that identifies a resource reservation for the IMS communication session. receiving, by an application server from a first electronic device, a non-"Internet Protocol Multimedia Subsystem Session Initiation Protocol", non-IMS SIP, message (220, 320) through an IP network, wherein the non-IMS SIP message (220, 320) includes: a request to initiate an IMS communication session through an IMS network between the first electronic device and a second electronic device via the application server; authentication information associated with the first electronic device for validating the first electronic device at the application server, a destination address associated with the second electronic device, and an indication that indicates an association between the non-IMS SIP message (220, 320) and a corresponding SIP message (222, 344a) for initiating the IMS communication session between the first electronic device and the second electronic device via the application server; a request to initiate an IMS communication session through an IMS network between the first electronic device and a second electronic device via the application server; authentication information associated with the first electronic device for validating the first electronic device at the application server, a destination address associated with the second electronic device, and an indication that indicates an association between the non-IMS SIP message (220, 320) and a corresponding SIP message (222, 344a) for initiating the IMS communication session between the first electronic device and the second electronic device via the application server; authenticating, by the application server, the first electronic device based, at least in part, on the authentication information in the received non-IMS SIP message (220, 320); initiating, by the application server, the IMS communication session with the second electronic device in response to the request included in the received non-IMS SIP message (220, 320); receiving, by the application server, the SIP message (222, 344a) that initiates a SIP INVITE session between the first electronic device and the application server, wherein the SIP INVITE session is routed through the IMS network corresponding to the received non-IMS SIP message (220, 320) through an IMS network (204, 206); associating, by the application server, the SIP message (222, 344a) received through the IMS network (204, 206) with the initiated IMS communication session; and sending, by the application server to the first electronic device, a SIP response message that identifies a resource reservation for the IMS communication session.