DIYbanter - 1980's model Southwestern Bell Desktop Phone-dead

My 1980's model Southwestern Bell Freedom Phone desktop phone was made
before quality and durability were completely forgotten by designers.
It has a nice accessible hookswitch with contacts visible for
inspection, and a really nice touchtone keypad that feels solid and
looks like it will last a long time.

This phone has three wires connected to the input cable, red green and
black, while all my other phones have only two wires connected to the
input cable.

I haven't used this phone in a few years and it seems to be totally
dead. There is no dial tone or static or line noise, no dialing will
work, no tones, nothing.

So far I've checked the input resistance with the hookswitch open and
closed and it gets a finite but very large resistance (8.5 Meg-OHM, I
think) with the phone off hook, between the red and green wires, and
something similar with the other pairs.
The black wire is actually connected to the hookswitch contacts and
also to a metal plate on the dialing keypad.

The piece you hold in your hand (handset?) will work when connected to
another phone.

What should I do next to diagnose the problem? I have perused this
group's topics, but I'm afraid I have little experience with phone
repair beyond cleaning the contacts on the hookswitch.

Thanks, Guy