SIP 481 Call/Transaction Does Not Exist

What SIP 481 means

A message arrived for a call the receiving system no longer recognises. It is the signature of SIP ALG mangling in-call messages, and it is a leading cause of calls that drop after roughly thirty seconds.

Client error (4xx)
Sent by The far-end server or handset
Common

What SIP 481 Call/Transaction Does Not Exist actually means

The server received a request that it cannot match to any dialog or transaction it knows about. Mid-call messages carry identifiers that must match exactly, so anything that rewrites them in flight makes perfectly valid messages unrecognisable to the far end.

What causes SIP 481 on a business phone system

SIP ALG is rewriting in-call messagesThe dominant cause. A router that alters headers on the way through breaks the match between a mid-call message and the call it belongs to, and because it usually only affects messages after setup, calls connect normally and then fail.
The session timer refresh is not getting throughWhere a system expects a periodic refresh to keep the call alive and that refresh is lost or altered, the far end tears the call down and then rejects anything further about it. Calls dropping at a consistent interval such as fifteen or thirty minutes point here.
One side already ended the callA lost hang-up message leaves the two ends disagreeing about whether the call exists, and the next message about it is rejected. Usually harmless and self-correcting.
An ACK never arrived, so the call was never fully establishedWhere the final handshake message is lost, the far end abandons the setup while the caller believes the call is up. Audio never starts and the call ends shortly afterwards.

How to fix SIP 481

Disable SIP ALGThis resolves the large majority of 481 faults. Check the router, any separate firewall, and the ISP-supplied device even if you think you are not using it for voice.
Check the session timer settingsWhere calls drop at a consistent interval, compare the session expiry on the PBX and the trunk. Aligning them, or disabling session timers if your provider does not need them, stops the periodic teardown.
Look at the timing of the failureCalls failing around thirty seconds point at ALG or a lost handshake. Calls failing at fifteen or thirty minutes point at session timers. The clock is the most useful diagnostic you have here.
Move the phone system traffic off the consumer routerWhere a business depends on voice, a router that respects SIP is worth more than any amount of tuning. Our router checker lists which models expose an ALG setting you can actually turn off.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SIP 481

Why do my VoIP calls drop after 30 seconds?

A 481 at around thirty seconds is one of the most recognisable faults in VoIP, and SIP ALG on the router is usually responsible. The call sets up correctly, then a mid-call message is rewritten in transit, the far end cannot match it to the call, and the call collapses. Disable SIP ALG first.

What does SIP 481 Call/Transaction Does Not Exist mean?

A message arrived about a call the receiving system has no record of. Either something altered the identifiers in transit so they no longer match, or the far end had already torn the call down.

Related SIP response codes

Source. SIP 481 Call/Transaction Does Not Exist is defined in RFC 3261 section 21.4.19 and registered in the IANA SIP Parameters — Response Codes. The causes and fixes above are drawn from our own experience supporting UK business phone systems.

All VoIP & SIP codes