Unfortunately there system requires dialing "*701" or "*702" and dialing star on a true rotary phone isn't possible without some other system in place. Like this product from dialgizmo: https://www.dialgizmo.com/how_it_works.html Dialing two consecutive stars would still be annoying and slow.

I've been wanting to wire up an in-house "intercom" system using old phones. I'm a little surprised there isn't a "telephone HAT" for the raspberry pi that will interface directly with an old phone. A HAT that could interpret the DTMF or pulse dialing and drive even the old fashioned mechanical ringers.