In a message dated 9/21/99 8:23:05 AM Eastern Daylight Time,
foschid@SUMMER.ETRURIA.NET writes:

> I understand they call it a 2-wire/4-wire interface, but I never saw one.
>
>  Any hint ?
>
>  I guess I need some current to circulate for the POTS to work.
>
>  AFAIK 20mA is what is required. Being 600 ohm the standard resistance of a
>  POTS, 12 V DC will be enough, on paper. Right ?
>
>  I hear that the DC voltage supplied to POTS must be very clean.
>
>  How should I filter the 12V DC supplied my a cheap external power supply ?
>
>  Thank you
>
>  Duilio Foschi
>
Motorola makes a speech circuit for this application.  It is the MC34014P.
Works great.  You need a bridge rectifier for polarity guard and a few
resistors and caps.  It is also a good idea to put a TVS on the output of the
bridge rectifier to suppress voltage surges on the telephone line.

Hope this helps,
Max