An old rotary phone can be used for a number of purposes in your Arduino projects - use it as a novel input device, or use the Arduino to interface a rotary phone to your computer.
This is a very basic guide describing how to interface the dial to an Arduino, and get the number dialed passed into a computer over the Arduino's serial link.
Step 1: Remove the dial from the phone
First step is to remove the dial unit from the phone. I'm using a GPO phone of some sort from the 1970s.
On this phone, the dial popped straight out - I just needed to give it a tug. If it doesn't, you may have to open up the phone and work out how to get it off.
There were five cables connected to the back of the dial unit. On my phone, these were regular spade connections, so I loosened the screws and pulled them out. If you want to re-assemble your phone, remember to record which color wire goes to which connection.