1. A method for converting a plurality of unicast sessions to a multicast session using the Session Initiation Protocol (SIP), comprising: receiving, at an application server, a request by a user device for content available from a content server; in response to the request by the user device, determining whether a threshold number of unicast sessions exist for delivering the content to other user devices; and in response to a determination by the application server that a threshold number of other unicast sessions exist for the content: generating a multicast source establishment message adapted for causing a multicast server to operate as a multicast session source for delivering the content to the client devices using a multicast session, and propagating the multicast source establishment message toward the multicast server; generating a unicast session establishment message adapted for causing a unicast session to be established between the content server and the multicast server for delivering the content from the content server to the multicast server, and propagating the unicast session establishment message toward at least one of the content server and the multicast server; and generating at least one multicast session join message adapted for enabling the other client devices to join the multicast session and propagating the at least one multicast session join message toward a SIP server adapted for propagating the at least one multicast session join message toward the other client devices.