Disconnect supervision would be good.

I'm going to attempt to post the CO off hook and hold circuit using Jeff M's method. At the time I didn't have an electronic template for Power Point so the schematic has components drawn from shapes.

This allows using any analog phone on the system. I have a couple 2564 keyfields in a small wood enclosure with a modern speakerphone on top. When the line is idle the 48v turns the optoisolator on, sending a ground to the transistor keeping it off. When a phone is off hook there's not enough voltage to activate the optoisolator. That allows the transistor to activate via the 47k resistor as there's no longer a ground through the 390 ohm resistor. The CO seems to reverse T&R during the ring cycle - the lamp will remain on between ring cycles. I could have made a bridge rectifier, but the answering system will pick up on the second ring.

(Thanks Jeff, using preview it appears the image will display)

[Linked Image from i.imgur.com]