SIP 606 Not Acceptable

What SIP 606 means

The session as described is not acceptable anywhere for that user. It is the global counterpart of a 488, so the causes are the same media negotiation problems but the refusal applies to every device they have.

Global failure (6xx)
Sent by The called user agent, refusing on the user behalf
Rare

What SIP 606 Not Acceptable actually means

Some aspect of the described session was not acceptable, and the response is made on behalf of the user rather than for one device. In practice it is rarer than a 488 because most systems refuse per-device, and seeing it points at a deliberate policy on media rather than a mismatch.

What causes SIP 606 on a business phone system

No codec is acceptable on any of the user devicesA stricter version of the usual codec mismatch, applied across the account rather than one handset.
A media policy refuses the session typeWhere video or a particular media type is offered and the platform disallows it outright, the refusal is global.

How to fix SIP 606

Align codecs as you would for a 488Enable G.711 A-law as a common baseline at both ends.
Check whether the media type is permitted at allConfirm video or any other non-audio media is actually enabled on the account before troubleshooting further.

Fixing the underlying problem

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

VoIP audio codecs explained

Questions about SIP 606

What is the difference between SIP 606 and SIP 488?

Scope, exactly as with 486 and 600. A 488 refuses the media at one device, so another device for the same person might still take the call. A 606 refuses on behalf of the user everywhere.

Related SIP response codes

Source. SIP 606 Not Acceptable is defined in RFC 3261 section 21.6.4 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