SIP 415 Unsupported Media Type

What SIP 415 means

The receiving system does not support the format of the message body it was sent. In voice terms that usually comes back to the media description, so treat it alongside a 488 and start with the codec configuration.

Client error (4xx)
Sent by The receiving device or server
Occasional

What SIP 415 Unsupported Media Type actually means

The request was refused because the body is in a format the server does not support. It concerns the type of content attached to the message rather than its addressing, which is why it clusters with the media negotiation failures rather than with the routing ones.

What causes SIP 415 on a business phone system

An unexpected content type was sentWhere a device attaches something the far end does not handle, the whole request is refused rather than the attachment ignored.
The media description could not be interpretedA session description that has been altered in transit, commonly by SIP ALG, can arrive in a state the far end will not accept.

How to fix SIP 415

Check codec settings at both endsEnsure a shared codec exists, with G.711 A-law enabled as a baseline on UK systems.
Disable SIP ALGIt alters the message body as well as the headers, which is enough to make a valid description unacceptable.

Fixing the underlying problem

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

Questions about SIP 415

Is SIP 415 a codec problem?

Usually, yes. Strictly it means the message body format is unsupported, but on a voice call the body is the media description, so in practice it comes down to codec configuration or something rewriting the description in transit.

Related SIP response codes

Source. SIP 415 Unsupported Media Type is defined in RFC 3261 section 21.4.13 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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