Craig,

I assume you are not connecting to Telecom network as you need to build your
circuit to meet their approvals. We have commercial products that connect to
the phone network that are approved but cannot give you circuit details of
these.

If you simply wish to use a touch-tone phone as a DTMF source then the
simple circuit described below should suffice.

1) Put a 330R resistor in series with a 12V power supply to one input of the
phone terminals(pin 5 or 2 on the BT plug).

2) Put another 330R resistor from the remaining phone terminal to ground.

3) Put a capacitor between each terminal (5,2) to your 8870 device.

This now gives you a balanced output , 600 Ohm impedance and 12V supply for
the phone. You should hear the tones in the hand piece if it is working, if
not... try reversing connections to pins 5 and 2. (Usually these are RED and
White)

Regards
David Huisman
Orbit Communications
http://www.orbitcoms.com
NSW
Australia