Practical onstage old BT rotary phone /telephone ringer UK.  

Relatively low cost method to ring an old GPO / BT phone on stage for theatre productions.
For this I use a BT rotary phone. £20
A BT 286 a linesman's Phone £30
A 12volt rechargeable Lipo £30 - 40
Buck / boost converter 5 for £10
speaker wire
croc clips
ADSL filter 50p from a junk shop.
What isn't here is a long phone extension cable.
You can replace the battery and buck converter with a variable power supply for less money. it doesn't need to be portable.
an interesting mod might be to add dmx functionality to the 286 to create a regular ring, but if you were there in the olden days you can ring the phone realistically yourself.
Yes you can do it with Qlab, with a nice fade trigger to emulate the ringer fall off but you know it's not the same in your heart of hearts it doesn't matter where you hide the speaker.