3.1.5.4.2 Negotiation Failure and Error Messages

If the initiator requires encryption and then receives a 488 error response to INVITE, it is recommended that the application display a new error message in the user interface (UI) that explains to the user that the encryption cannot be negotiated.

The 488 error response SHOULD contain a Warning header to indicate that encryption levels are incompatible. The following is an example of a 488 error response.

 SIP/2.0 488 Encryption Levels Incompatible
 Warning: 308 ms.com "Encryption Levels Incompatible"