The Tele Q, made by CEI Inc, PO Box 51, Deborah, IA 52101 Tel: +1 319
382 0041, Fax: +1 319 382 0041 is one gadget to do this. Approximately
US$110-120. It's US$18 for a power supply, but batteries last a long
time.

Norcostco at http://www.norcostco.htm/ have it in stock for US$120 at
the time of writing

Maplin MPS, PO Box 77, Rayleigh, Essex, UK, +44 1702 554400 make a kit
called the 'Autoring', P/N LT19V. Maplin live at
http://www.maplin.co.uk/index000.htm - they list a number of overseas
distributors there.

Pricing anyone? It's an expensive call from New England.

19.95 (pounds sterling) in their 96/97 catalogue.

Jech Tech Inc, 13962 Olde Post Road, Pickerington, Ohio, 43147, USA
Tel: +1 614-927-3495, Fax: +1 614 927 3493
Sales & service : jectech@infininet.com
...make a small PCB module generating 180V pk to pk at up to 20 Hz,
ringing up to 5 REN (ringer equivalents). Frequency is adjustable for
non US phones. Requires 12V DC power supply. US$49.95 plus shipping and
handling. They have a web site at http://www.infinet.com/~jectech/

http://www.hut.fi/~then/circuits/telephone_ringer.html has general info
on ringing telephones along with several means of producing ring
voltage. Links to commercial equipment sources and to scratch built
plans as in the Wenzel link below

http://www.wenzel.com/pages/mystrylb.htm has complete plans for a phone
ringer providing ring voltage and cadence control, provisions for talk
circuit and audio input. It's in PDF format so you'll need Acrobat from
http://www.adobe.com or xpdf from your favourite archive. Looks like c.
US$20 component cost.

In the UK, phones are rung with 50V A.C., at 25Hz. If want to ring a
phone where the clapper oscillates between 2 bells, remove one of the
bells, and run it from a transformer giving 50VAC, 50Hz. If you want to
ring a more modern phone, a lot generate the ring frequency themselves,
which makes it easier. Get hold of a master socket (the type with the
surge arrestor, out of service resistor and a capacitor inside), and
apply 50VAC 50Hz to the terminals A and B, and the phone will sort out
the frequencies itself.

[Thanks to Murr Rhame for most of this info]

Abbagail Winters tells me that in australia, Telstra techs can often be
talked into giving a community theatre company a few of the old decadic,
rotary dial phones, a transformer (dc 28v?), and the necessary info on
pinouts to make a phone ring.