SIP
481 Call/Transaction Does Not Exist
The UAS received a request that does not match any existing dialog or transaction. The call state may have been lost.
When You See This Code
When sending a BYE, re-INVITE, or UPDATE for a call that the server has no record of. Common after server restarts.
How to Fix
The dialog has been lost. Terminate the local call state. For re-INVITE failures, re-establish the call from scratch.