I'm trying to connect an old phone's three wires (white, red, green) to my home line VOIP-4-wire (black, red, green, yellow) but I don't know where or what to connect.

Phone electrical circuit image

Phone wiring diagram image

I have read this:

I don't know much about electronics so any help is good.

  • 1
    \$\begingroup\$ Incompatible without a redesign. \$\endgroup\$
    – Andy aka
    Commented Dec 27, 2022 at 11:14
  • \$\begingroup\$ @Andyaka Probably. It depends on whether the VoIP equipment generates the ringing current required by vintage telephones. See vintage-radio.net/forum/showthread.php?t=62 for various descriptions of this. \$\endgroup\$ Commented Dec 27, 2022 at 14:14
  • \$\begingroup\$ Unless you can hum DTMF it probably wont be able to make outgoing calls. \$\endgroup\$ Commented Dec 28, 2022 at 4:42

1 Answer 1


enter image description here

The section in red are the terminals of your photo. The section in blue represents the house wiring. Note that this arrangement allowed for external connection of the bell circuit by connecting BT2 to BT3 as shown for 2-wire connection. This arrangement was due to the typical telephone exchange bell ringer circuit limit of three bells on a line. (Most phones had a Ring Equivelant Number (REN) of 1 but other devices such as modems, fax machines, etc., may have had more or less.)

Try looping T17 to T18 in the phone and just connect red and green to your VOIP 4-wire. If the ring voltage is sent out with the voice on those two wires it should work. If not, then report back and give a link to the user manual for the VOIP interface.


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