It depends what has actually locked up. E.g., if it is a SIP call, it could be the phone.
If an Asterisk channel has failed to hangup, you should be able to use the CLI to clear it, but if it is the device level that is still ringing, but the channel is not up, that may not be an option.
You can do a ‘core show channels verbose’ or ‘core show channels concise’ on the Asterisk Console
Once you identify the channel you need to kill you should be able to do either a 'soft hangup ’ or 'hangup request ’ depending on your asterisk version.
Note that ring trip is done within the destination device on with SIP, so if picking up the phone is not terminating the ringing, that is definitely a device lock-up problem.
Rebooting the phone didn’t work. The phone can pick up other calls, however in FOP we still see it ringing. It does not actually ring the phone. It just appears as ringing in FOP, and records are being created in cdr.
I tried “soft hangup”, but I still get a “No such command” error.
As I said before, the CDR implies the channel was about to terminate. At the moment it seems likely that the device state has got upset, and there is neither an Asterisk channel (which you can hangup from the CLI) or a SIP channel, associated with it.
In that case, the only remaining option is to delete and re-add the device. If you are using sip.conf, removing it from the file, then doing sip reload, should not disturb any other sip device.
There is a slight risk that you have a limited deadlock, in which case the sip reload might hang.