SIP 501 Not Implemented

What SIP 501 means

The server does not support the thing you asked it to do at all. Unlike a 405, which refuses a method for one address, a 501 means the capability is simply not implemented, so the fix is to stop asking rather than to change a permission.

Server error (5xx)
Sent by The server that does not implement the request
Occasional

What SIP 501 Not Implemented actually means

The server does not support the functionality required to fulfil the request, and it is the appropriate answer when a request method is unrecognised entirely. Because it describes a missing capability rather than a policy, retrying or adjusting credentials achieves nothing.

What causes SIP 501 on a business phone system

A feature is enabled on your side that the far end does not supportPresence, message waiting, in-call information messages and call recording controls are all commonly unsupported on gateways and older handsets.
A gateway or ATA does not implement the requestAnalogue adaptors implement a deliberately small part of SIP, so anything beyond basic calling can return a 501.

How to fix SIP 501

Identify which request is being refusedTake a SIP trace and look at the method in the failing request. That names the feature to switch off.
Disable the unsupported feature for that deviceTurn it off per device rather than globally, so handsets that do support it keep it.
Check whether firmware adds supportSome capabilities arrive in later firmware. Worth checking before replacing hardware.

Fixing the underlying problem

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

Questions about SIP 501

What is the difference between SIP 501 and SIP 405?

A 405 means the far end understands the request but will not allow it for that address, and it tells you what it will allow instead. A 501 means it does not implement the capability at all, so there is nothing to permit and the only fix is to stop sending it.

Call hangup causes that become SIP 501

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 501 Not Implemented is defined in RFC 3261 section 21.5.2 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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