[Serusers] SER Bug? 503 messsage is not handled properly

[Pat wang]

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