To get you started,   The open circuit voltage is 48 volts (called on hook), and
 the off-hook voltage is about 6 to 15 volts, depending on the telephone used.
A typical 600 Ohm phone should yield about 12 volts when off hook.  The ring vol
tage is about 90 volts AC, the frequency can vary depending on the system (The d
ifferent frequencies used to be used to select different phones on a party line)
.  The caller ID information is sent between the first and second rings as an f
sk modulated modem signal at 1200 baud, as i recall.   The dial tone is made up
of two very accurate sine waves mixed together, so its sounds the same everywher
e and is easy to detect electronically.

You are not supposed to hook un-registered devices to the phone line (FCC regula
tes this).  Most phone interface circuits isolate the line with a 600 to 600 ohm
 transformer, go off hook with a relay contact, and optically isolate the ring d
etection, so that no DC connection exists between the line and the customer equi
pment.  Be careful not to accidentally connect the phone line to a power supply
or AC.  This could be vry very expensive for you.

 Regards,
   Ron Fial
===================================================================
At 07:45 PM 7/9/98 -0600, you wrote:
>Sorry to bother you again with another OT.
>
>Does anybody know a link where I can find info on the US telephone system
>specifications?
>
>Actually, what I need to know is the voltajes that must be present on the
>lines, and the signal levels.
>
>Calvin
>