R4+Z

I posted this on another site under another name (can't use this one there for some reason) and it may be worth a try.

1/ There is a command which you won't find in the command manual (it's hidden away in another manual) which can be used to forcibly release a station or trunk. I have found that this can often get rid of some of the quirkier problems as it seems to reset the len back to a starting point.

The command is EB2 de stn number de ccc exe
(EB0 trunk number for trunks)
note for a digital you must prefix the station number with F

The way to use it is set up a call to or from the station in question run the command then wait for the call to be closed by the system it may take a minute or two.

See how you go.

R4+Z

Just another point worth trying is see if there is a message/reminder set. You will have to find out what code is set to A146 in cmd 200 or set a code to that value. Then at the offending phone dial the code followed by 3, you may have to do this more than once as there can be multiple messages set.