The 2500 has a network block (potted bolus of electronics that make it tick) with all of the wiring we need attached via screw terminals.
You'll disconnect some of these wires to connect the switch hook and receiver earpiece to the Feather.
Locate the wires running to the earpiece from the RJ9 jack. You can unscrew the handset mouthpiece or earpiece to see which wires run to the speaker, in this case, green and white wires.
Unscrew those two wires from the phone's network block and screw them into the terminal block on the amplifier board.
Switch Hook Connection
Use a multimeter to find two connection points on the network block that are opened and closed when the switch hook is engaged.
Run two blue wire leads with ferrule connectors on their ends to these two points and screw them in place. (Fragile bits of plastic snapped off on mine, hence the Kapton tape).
Use the Euro block connector to connect the two blue switch hook wires to the two black En/GND wires.
You'll run USB power to the Feather in order to charge the LiPo battery when needed.
Prepare a black jumper wire and a red jumper wire with ferrule ends that can be used in the Euro block on one end and DuPont connector pins on the other (premium silicone jumper wires work well for this).
Run the header end of the USB breakout cable through the RJ11 jack, then connect the two power extension wires. You can ignore the USB data lines.
Connect the power and ground lines to their respective leads on the Feather Doubler using the Euro block.
The receiver and USB power are now connected to the Feather, which is plugged into the keypad. You should test the following:
- switch hook works to enable/disable the Feather
- audio comes through the earpiece speaker
- keypad entry is properly registered
- USB charges the battery
You can now start re-assembling the phone.
Place the shell onto the base, making sure the RJ jacks fit their respective places.
Tighten the two captive screws in the base.
Using the phone is simple! Pick up the receiver, wait for the dial tone, and then call one of the seven digit numbers in the directory.
The phone will ring a randomized number of times and then "pick up", playing the song you wanted!
If you dial a wrong number you'll hear either a busy signal or the "not in service" message.