As with every solution comes a problem.

This project was to solve a problem with an elderly neighbor not hanging up their cordless phone by pressing End Call, or placing in the charging cradle.

It will perform that function, but here is the other problem, this neighbor takes the phone OFF HOOK on purpose so as not to be disturbed by incoming calls  while taking a nap.  This is hopeless.

This whole project could be implemented by the Cordless  Phone Manufacturer using their existing hardware with software changes. They probably monitor the modulation level between the base station, and the cordless phone, and they certainly monitor the Off hook condition.

Thru software changes and probably no hardware changes the Cordless Phone Manufacturer could reset the offending OFF HOOK phone with no audio, and
could add a do not disturb for an hour or more, or for a specific time while routing incoming calls to the answering machine with the ring turned off and the message audio turned off.