If you look at the compatibility list on the Plantronics website, most of the current Panasonic GAP DECT 'phones are not listed. There are none of the KX-TG ranges shown there.
My 'phone system is KX-TG8120E, but I took the gamble of buying the C65 anyway, thinking that Plantronics had perhaps just not tested it with the latest Panasonic ranges.
But I just came off the 'phone to their (helpful - add a star for this !) support department, and they confirmed that there is a compatibilty issue with some of the latest Panasonic 'phones, including mine.
The compatibility issue may or may not be too serious depending how you want to use it. It just doesn't ring on incoming calls. But it will ring when trying to transfer a call (or as an intercom, same thing really). If you can hear the bell of another handset ringing, you can still pick up an incoming call on the C65. But it's useless if you wander away from other 'phones, you won't know when someone calls you.
Plantronics blame the new Panasonic 'phones for this small problem. You could say "they would, wouldn't they", but in this case I suspect it's true !
Apart from that it's o.k., sound quality isn't exactly sparkling (unlike the Panasonic system which is spectacular), but it's perfectly usable.