W56H handset no tones or ring

[swdatanet]

Hi
We have a W60B base and 3 W56H handsets connected. One of the handsets suddenly stopped ringing and at the same time it was noticed that the sounds when pressing keys also stopped working. the phone works fine otherwise and if you see the screen light come on when a call comes in then you can still answer. Voice volume is fine and you can hear and be heard when using it.
Tried all the settings to make sure advisory tones are enabled and volume is at maximum. Also checked that it is not silenced by long press on * key and that it is not on DND or offline etc. Have tried a handset reset to default, but that didn't work either. Taken the battery out for hours, but cannot make this handset ring. Firmware is on latest 61.85.0.20.
Anybody have any suggestions, is there a hard reset that I have missed, or is the phone faulty?
Thanks