Any good old analog phone will do, the bigger the better. Using an Arduino & a wave shield, you can make the phone behave any way you want & even fool someone into thinking they have reached an actual number and they are listening to a voice-mail.
This project can be done in a few hours and for very few dollars.
Step 1: Gather the materials
We used 1 Arduino Uno & 1 Wave Shield for the first phone. With a 9 Volt battery, you have power for a very very long time.
The phone original keyboard was defective (didn't want to spend too much time to find replacement keys/fixing it) so we had to substitute it with a Membrane keypad in this case.
Step 2: The Hack...
This exploit is made possible by simply cutting from the phone PCB the white & green wires that go to the handset speaker and connecting them to the speaker output of the Wave Shield.
Then, you cut one wire (I used Black) of the 9 Volt battery connector (somewhere in the middle) and solder each end to each side of the handset release connection (how the phone knows if he is off-hook or hangup). This will prevent any power to be used when the phone is NOT in use.
Like I said, in this case, it was easier & quicker for this application to simply add a membrane keyboard than fixing the original AT&T keypad.