To deal with ANAT/IP version incompatibility, a communication endpoint registers with a communication system using a SIP REGISTER message. For example, the communication endpoint registers when it boots up. The SIP REGISTER message indicates that the communication endpoint: does not support the Alternative Network Address Type (ANAT) protocol or is one of IPV4 intolerant or IPV6 intolerant. When a SIP INVITE message is received with a Session Description Protocol (SDP) offer that comprises the ANAT protocol or an IP address that is opposite of the IP version intolerance of the communication endpoint, a 4XX SIP response message is sent to the sender of the SIP INVITE to indicate the incompatibility. In response, to receiving the 4XX SIP response message, the SIP INVITE is modified to be compatible with the capabilities of the communication endpoint.