From: William Levak (wlevak_at_cyberspace.org)
Date: 2004-01-31 00:20:40
Dialing a phone interrupts the dial signal from the phone company ONLY on
old rotary dial phones.
Newer push button phones (and modems) use a relay to interrupt the signal,
but they also have an isolation transformer between the relay and phone
line, and may have other circuit components as well, depending on the
features of the phone or modem.
Touch tone phones use an oscillator circuit to generate the tones. The
phone company's "dial" power is used to power the oscillator circuits.
There is not necessarily any relation between the power source and
the oscillator output power. This is dependent on the design of the
dial circuit and what it is expected to be connected to. There is also an
isolation circuit between the oscillator and the phone line.
The voice circuit runs directly off the phone company's power ONLY on
old rotary dial and early push button phones.
Newer phones and directly connected modems use the "voice" power from the
phone company to power voice circuits in the phone or modem (usually
integrated circuits). Again, the output of the voice circuits is
dependent on the design of the circuit. This voice signal is coupled to
the phone company's voice signal through an isolation circuit.
Summary: Only old phones directly operate off the phone company's
signals. Newer phones, and all directly connected modems, use the phone
company's signals as a power source, and then couple their output back
onto the phone company lines through an isolation circuit.
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