The KX-UT670 has both an EHS (electronic hook switch) and 2.5mm headset port on the side.
Being that the Plantronics CS70n is one of the supported headsets I hooked it up to the phone using the following setup:
Plantronics 3.5mm hook switch port to the Panasonic 3.5mm EHS port
Plantronics RJ9 to Panasonic 2.5mm headset port
When pressing the “hook” button on the Plantronics headset the phone would answer and disconnect after a second.
If the hook switch wire is disconnected the headset works and if the headset wire is disconnected the
hook switch works but it wont work together.
I sent an email to Panasonic and they sent me back the following fix:
The phone doesn’t support both the EHS and 2.5mm headset at the same time.
To use the EHS with the Plantronics headset you must disconnect the handset from the phone, plug a RJ9 cable from the phone to the Plantronics and then plug the handset into the Plantronics.
See the following Power Point Link for the exact setup.