[dagskarlsen]

You are right.
I would like to see that relay following 20-25 hz.

I have without asking, modified Terrys diagram.
If you have some kind of 25 hz power (15-30 hz) it will be the best:

This could be an old ringing machine (frequency dividing unit) or a frequency converter and a transformer.

If not, 50 hz will actually ring many telephone ringers, I have an Ericsson PABX ringing at 50Hz 45V, it sounds a little bit odd.

The rectifier bridge will protect the CO equipment from high ren loads, and strange feedback from the unit.

The 3rd relay contact set solves the need of ringing on 2 wire phones.

Relay contacts 1 and to may be moved to primary side of the transformer, and if you only have a rely with 2 set of contacts, you may skip one of these.

What is the drawback putting in this 3rd contact set? When picking up the handset during ringing, the ring current will continue until the ring is finished, and the voice circuit will be given more current than expected. The 1.8 microfarad capacitor will limit the current.

dsk