I have a test setup like: UAC1-----SER------Third Party Proxy----UAC2 UAC1 registers to SER directly while UAC2 registers to SER via a third party proxy. When UAC1 calls UAC2, the third party proxy replies a 100 trying then 503 Service Unavailable to SER. There is no Retry-After in the 503 message. SER keeps re-sending INVITE. By RFC, If no Retry-After is given, the client MUST act as if it had received a 500 (Server Internal Error) response. Is this a bug on SER? Thanks, Pat