Well... I'm pretty sure that you don't want a phone that just plays hold music...

But there are countless other exciting projects you can make with this very basic hack of these readily available "desk" phones.

Excited to see what other projects come out of this Instructable : )

Have fun!

Step 1: Parts Parts Parts

Parts:

And of course, an old phone! For reasons I can't quite understand there are many cheap options for phones like this on Amazon. Who is buying them, and why? Here are some Amazon options:

Old phone A (used in this hack)
Old phone B

Total cost for this project will be ~130$

Step 2: Check It Out

Open it up.

We can throw out the bell.

See that little white plastic lever on the circuitboard? That is what detects if the phone is hung up. We are going to use that because it is quite annoying to build our own switch for this.

Also note the color of the wires which connect to the handset speaker: red and green. (yellow and black are for the mic).

Step 3: Install Our 3W Speaker

In replace of the bell, glue in your 3W speaker.

I cut away the supports that were in the way.

Step 4: Intercept the Hangup Trigger

These are the points that we need to solder on the existing PCB to intercept the hangup switch. I connected my two green wires to them.

Don't worry if you have a slightly different phone. All phones will have a switch similar to this and you can use a multimeter tool to detect which pins go HIGH/LOW when the switch is pressed.

Step 5: Build the MP3 Shield

Great instructions on the Adafruit page for building your MP3 Shield: https://learn.adafruit.com/adafruit-music-maker-shield-vs1053-mp3-wav-wave-ogg-vorbis-player

Remember to close the 3pins that enable it to be used on an Arduino Mega.

And I also closed the pins that give it a +12db sound boost. It sounds like hell once these are closed, but whatever, this is hold music, right?

Step 6: Intercept the Keypad

Thankfully on my keypad the rows and columns were numbered on the circuitboard.

Sometimes keypads have 8 wires. Sometimes they have 7.

Make note of these numbers and what Arduino pins they are plugged into. You will need to enter this into the code later.

Step 7: Intercept the LED

Well, sadly, the LED on my phone was bust. But I am sure it works on other phones. Why not put it to use? You may want to add a resistor because I didn't see one on the tiny circuitboard the LED is mounted on.

Step 8: Install Our Proximity (Motion) Sensor

We use the proximity sensor to detect if someone is standing in front of the phone, and if so, it starts ringing. This is a great way to get people interacting with your phone without you being there. Who can resist an ambiguous bright red ringing phone? The answer is, no one.

Drill a hole carefully (start with a small bit and increase size). You can mount these ultrasonic rangefinders on the back side and even cover them up a bit and they will work just fine. I added a small acrylic piece for cosmetic purposes but if your drilling is good you shouldn't need it.

Step 9: Connect the Audio: Speaker and Handset

Left goes to the handset. Right goes to the speaker. Don't mix them up!

Step 10: Add the Tuning Dials

These are not essential but nice to have in case you want to tune some variables without having to re-upload the code every time.

I added two tuning dials. One for the distance (proximity) at which the phone is triggered.

The second is the duration before it starting ringing again after hung up.

Step 11: Get 'Em All in There

There is a very convenient screw hole that allows you to screw in the Arduino Mega right in between the speaker and the existing circuit board. It's like this phone wants to be hacked, right?

Also I spliced the power supply and soldered it directly to the Vin on the board. This saves space a bit.

Everything fits in there nicely.