My original intention was to create a sort of presence detector for an outside door. With two PIR detectors it could determine if someone from outside was entering the room, but would be unaffected by activity in the room itself, and then, if detected, sound a standard GPO type ring. https://www.youtube.com/watch?v=4R2i6qar79g so as not to alarm the "intruder" who would simply think the telephone was ringing. The basic circuit works but I still have to design a base unit including batteries, a charging part and the PIR detectors. I could (just) have included all that in the bell box but I did not want to drill more holes in that original box.
Anyway, this is another of my projects which is currently pending the allocation of more resources.
If you do decide to build exactly that project take note of the errata in the body of the description.