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