481 4xx Client error RFC 3261
SIP 481 Call/Transaction Does Not Exist
The server received a request (BYE, re-INVITE, CANCEL) that does not match any existing dialog or transaction.
Typical Q.850 / ISDN cause: 41 (Temporary failure)
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Common causes
- The call has already ended on the far side (often after a timeout).
- NAT or a SIP ALG changing Call-ID, tags, or Contact.
- A failover SBC that doesn't share dialog state.
How to fix it
- Check whether the call dropped earlier because of a missing ACK (calls dropping after about 30 seconds).
- Disable SIP ALG, and make sure Record-Route and Contact are correct through NAT.
- For HA proxies, replicate dialog state (for example, OpenSIPS clusterer).
Seeing 481 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 4xx responses
The request has a problem or cannot be completed at this server, usually because of something on the requesting side or the target user.