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.