Here is a company that makes a ring generator. **broken link removed**
In the US, there were different ring frequencies used in "party line" configurations. This is where many people share the same line, but the ring signal changes to ring the specific houses. I found this one
http://www.camblab.com/home.htm using "telephone ring generators" for a search.
Telephone simulators are traditionally expensive because of DTMF and Caller ID and measuring the stuff that makes the phone system work.
REN (Ringer Equlivelency Number) determine how many bells a line supports. The REN's are added.
The BT system should be a lot easier to inbterface because the ringing signal is separate. I believe there is a capacitor in the jack.
Modern ringing interfaces (detection) are done with optocouplers.
You may sfind some stuff using "Data access arrangements" or DAA for a search.
http://www.google.com/url?sa=t&rct=...Gu44Fw&usg=AFQjCNEIMJBFKBsN71KINR-h60nPS-b8PA
I had in my bookmarks, (lost) a device that would ring the line whan the phone was picked up.
I can't find what I'm looking for:
http://www.telephonetribute.com/telco_line_simulators.html
There is a device that when the phone is picked up the other one rings. Here is one: **broken link removed**
BUT remember, the BT system is different than the US and BT is a lot more restrictive than the US. The ringing signal being separate is the major difference.
In the US, we are responsible from the demarcation point (called the NID or Network Interface Device) which is usually mounted to the exterior of a residence. We get to maintain the inside wiring. Early (1950's) houses were wired with quad wire and later power was supplied on two of the wires for lighted dial phones (Princess).