Abstract: A SIP adaptor modifies the format of a session initiation protocol (SIP) message. A SIP message in a first format is received by the SIP adaptor from a first SIP server that is based on the first message format. The SIP adaptor modifies the SIP message to a second format and forwards the modified SIP message to a second SIP server that is based on the second message format. By modifying SIP messages, the SIP adaptor allows communications sessions to be established between clients of SIP servers that utilize dissimilar SIP message formats.