Hi slm265 thanks for your post here in the Community although we're sorry to hear of the concerns you've raised.
So any additional handsets would need to directly connected into the 21CV network, via the adapter or an extension so this is likely to be an incompatibility issue of the handset itself.
We'd advise contacting Panasonic to see if they can shed any light on this further, but for an extension, it is likely we would need to run additional wires from your Hub to the new device itself and we can help with this if you require it.
We do hope this helps and we're here if you need us.
Many thanks