SIP
505 Version Not Supported
The server does not support the SIP protocol version that was used in the request. Currently only SIP/2.0 is standardized.
When You See This Code
Extremely rare. Would occur if a client sends a request with a SIP version other than 2.0.
How to Fix
Ensure your UA uses SIP/2.0 in the request line. This is the only standardized version.