On Thu, 18 Dec 1997 15:37:40 EST Harold Hallikainen
<haroldhallikainen@JUNO.COM> writes:
>        People considering connecting stuff to the PSTN should also
>have a look at part 68 of the FCC rules (47 CFR 68).  You can view it
>at http://hallikainen.com/FccRules  .

The devices that were sold to connect to an older telephone to stop the
bell from ringing were bridge rectifiers.  Since the bell is AC-coupled
to the phone line, sending only DC to the phone prevented the bell from
ringing.  However, the phone functions were not affected (other than
having 2 diode drops less voltage to operate from).

Arrangements to clamp the voltage will likely not work very well,
up to the point of preventing normal operation or even making the phone
company think your line is faulty and temporarily disconnecting the
service.