Powering an External Phone Ringer - Straight Talk Home Service
For quite a while, Sparkfun had a series of wireless desk phones with the circuit housed in an old Bell System 500-style case.  So you could have an old-style black desk phone with a rotary dial and old-fashioned ringer, powered by a Li-Poly battery and connected to the telephone network by a cellular modem.

Since there was no phone line to activate the ringer, they built their own high-voltage circuit.  These phones have been retired from the Sparkfun catalog, but per their policy the information is still available including schematics and datasheet.

Here is a link to the schematic.  The high-voltage ringing circuit is on page 3, which I have copied below: