The Panasonic KX-TPA60 DECT cordless handset available with the TGP600CE system has a 2.5mm diameter plug socket. The very best headset for the KX-TPA60 I have tried is the Sennheiser IMPACT 600series. The plug of this HS is 3.5mm, there will always be an adapter available for 2.5mm sockets without any loss of quality.

I bought the Panasonic RP-TCA430 headset because it is one of the few headsets with a 2.5 mm TRRS plug. You can plug it in on the left side of the handset.