How would I detect if a POTS telephone is on hook?
A POTS telephone presents a DC resistance of about 180 ohms when it is off-hook, and open circuit when on-hook (the ringer bell is in series with a capacitor, blocking DC.)

Actual telephone lines use a sensing voltage of about 48V through a fairly high resistance. When the handset is lifted, the speaker and mic are connected across the line and draw current (20mA or so), and the voltage drops to 6 ~ 9V. The head-end senses this and changes over to the voice connection.

In your case you could use a smaller sense current - maybe just a 5K pull-up to Vcc - to detect switch closure (off-hook).

Here's a paper describing how POTS lines work: https://www.hermonlabs.com/Products/innerData/pdf/Analog%20Telephony%20Overview.pdf