Disclosed are a method, apparatus and system for interworking between H.320 or H.324 and SIP. The method comprises receiving a SIP message indicative of capabilities supported by a first endpoint device and deferring responding to the SIP message. The method further comprises receiving from a second endpoint device an H.320 or H.324 message indicative of the capabilities supported by the second endpoint device in response thereto responding to the SIP message thereby to establish a media communication channel between the first and second endpoint devices based on their respective capabilities.