The basic telephone operation shown in Figure 4
consists of a microphone and speaker connected together
in series with a battery. In this "telephone," when switch
S1 is closed, direct current (DC) can flow in the
microphone and speaker which moves the speaker cone a
bit but produces no sound other than an initial "pop."
However, when someone speaks into the microphone
(MIC 1), alternating current (AC) is produced which moves
the speaker cone and "roughly" reproduces the sound
spoken into the mic.
As an enhancement, I added an LED with a current-limiting resistor to show when a call was coming in. This is
only a slight improvement over the old can and string
acoustic phone, but a far cry from modern telephones
which use computerized switching, satellite relays,
terrestrial microwave links, wireless, cellular technology,
and digital encoding.
The schematic for an old line powered phone is
shown in Figure 5. This phone receives a 48 volt DC
signal from the central switching office to power the
phone. To ring the local phone, the central office sends an
AC signal of 40 to 150 volts with a frequency of 20 to 40
Hz (cycles per second) superimposed on the 48 volt DC
signal. When the receiver is lifted to answer a call, the
ringer signal is disconnected and the line voltage between
the local phone and the central office drops to eight volts
DC (these voltages vary among the different carriers).
[NOTE: At my home, the on-hook voltage is 52. 4 VDC
and the off-hook is 10. 9 VDC.] To make a call, the off-hook voltage to the central office triggers a dial tone so
the caller knows the call connection can be made. Most
phones also include additional capacitors to cancel out
the inductive reactance of the incoming phone lines.
The multi-line phone is just a normal telephone with
multiple lines running to it. A four-line phone has four
different lines run by the phone service provider and has
the capabilities to switch between lines, put a line on hold,
transfer a line, etc. An analog phone uses the amplitude
modulation (amplitude of voltage on the line varies as the
input voice signal increases) of the 48 volt DC line signal
to transfer voice signals versus the digital phones which
use digitally encoded signals to modulate the 48 volt DC
The analogy between analog phone and AM radio
signals is the radio has a high frequency local oscillator
signal which is modulated by the input signal, whereas the
phone modulates the DC signal. Thus, a four-line phone is
essentially four individual phones in one body. I can
imagine using several different frequency bands
(frequency division multiplexing) on the two-wire pair to
transmit several phone signals simultaneously with filters at
the receiver end separating the different "lines," but I do
not know of any phone service providers who do this on
land lines for voice service to residential users.
FDM is used on long distance trunk lines, satellite
communications, terrestrial microwave services, and
broadband DSL. This could be a challenge for our readers:
Design a multiple frequency/multi-line "phone" system for
use in a home or office, but not connected to the phone
company’s lines (see Q&A SIDELINES for more details).
Now, let’s answer your question on using an unused
line to indicate that you are using your VoIP line. The easy
way would be to hook your 5 VDC VoIP phone signal
directly into the four-line phone's unused line to see if it
worked. However, the easy way is not always the best way
since you could damage either or both phones, plus the
phone service providers require isolation of devices
connected to their networks. First, use my homebrew test
device shown in Figure 6 to check the off-hook voltage on
the four-line phone.
Hook the test device into the line you are going to
test with a short piece of telephone cord with RJ-11 plugs
on each end. Most four-line systems use the four-wire
phone cable to provide conductors for two lines per cable
(red-green on one line and yellow-black on the other), so
you will have to determine which line is which to obtain
the desired results. Hook a DVM to the red-green or
yellow-black pairs, punch in the line you are testing on the
four-line phone, take the phone off the hook, and check
the off-hook voltage that your phone produces. Also
check the off-hook voltage of your VoIP phone in the
January 2016 11
■ FIGURE 4.
■ FIGURE 5.