SIP
416 Unsupported URI Scheme
The server cannot process the request because the URI scheme in the Request-URI is unknown to the server.
When You See This Code
When using a URI scheme (e.g., tel:, im:, pres:) that the receiving server does not support.
How to Fix
Use a supported URI scheme. Most SIP servers support sip: and sips: schemes. Check server capabilities with OPTIONS.