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
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.
Step 2: Identify the switch
For those of you who may never have used a rotary dial before - remember that the dialing only happens when you let go the number and let it spool back .
I've documented how it works for my phone in the Notes of the photo below.
There's also a blurry video of the mechanism working.