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I'm guessing you're looking at a short loop application ( < 2Kft ). I'm currently using the Silicon Labs Si3210 SLIC. http://www.silabs.com. It has a built in controller for a DC-DC converter to generate the ringing voltage. I also noticed you asked about DTMF decoders in another post, the Si3210 has DTMF detection built-in. It also has a couple of oscillators you can use to generate dial tone and stuff.. Cost about $9 in low qty.