SIP 488 Not Acceptable Here

What SIP 488 means

The call was acceptable but the audio arrangement was not. A 488 is a media negotiation failure, so look at codecs, encryption settings, and whether SIP ALG has rewritten the media addresses.

Client error (4xx)
Sent by The called device, when it cannot agree on media
Common

What SIP 488 Not Acceptable Here actually means

The request was understood but the session description was not acceptable to this device, typically because there is no shared codec or the media parameters cannot be supported. It differs from a 606 in scope: a 488 refuses at this device, while a 606 refuses on the user behalf everywhere.

What causes SIP 488 on a business phone system

No codec in commonWhere one end offers only G.722 and the other supports only G.711, there is nothing to agree on. Common when a handset default is narrowed to a single codec, or when transcoding has been disabled on a trunk.
SIP ALG has rewritten the media addressA router altering the session description can leave an address the far end refuses or cannot use. The tell is that the same handsets work on a different connection.
Encryption settings do not matchWhere one side insists on encrypted media and the other offers it unencrypted, negotiation fails. Appears after enabling TLS or SRTP on one end only.
The offered address is a private one behind NATA device advertising its internal address gives the far end nowhere reachable to send audio to. Often paired with one-way audio on calls that do connect.

How to fix SIP 488

Line up the codec lists at both endsEnable G.711 A-law as a common baseline, since it is effectively universal on UK systems, and keep at least one shared codec everywhere.
Disable SIP ALGIt interferes with the session description as well as the headers, so it belongs on the list for any media negotiation failure.
Make the encryption settings matchEither both ends use encrypted signalling and media, or neither does. A half-configured migration to TLS and SRTP produces this exact failure.
Check the NAT and media settings on the handsetEnsure the device advertises its public address, using STUN or the provider recommended setting, rather than its internal one.

Fixing the underlying problem

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

Questions about SIP 488

What does SIP 488 Not Acceptable Here mean?

The far end was willing to take the call but could not agree on how to carry the audio. In practice that means no shared codec, mismatched encryption, or a media address that something rewrote in transit.

Why do I get 488 on some calls but not others?

Because different destinations support different codecs. Calls that stay on your provider network may share a codec while calls out to another carrier do not. Enabling G.711 A-law everywhere gives you a common baseline that almost eliminates this.

Call hangup causes that become SIP 488

If the call crossed a gateway between SIP and ISDN or SS7, this status may have started life as one of these Q.850 cause values. The mapping is defined in RFC 3398 section 7.2.4.1.

Related SIP response codes

Source. SIP 488 Not Acceptable Here is defined in RFC 3261 section 21.4.26 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.

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