Here is an amazing step-by-step by Adafruit.
** Remember to close the three jumpers so it will work with an Arduino Mega **
The shield has 3W outputs, which work perfectly for the speaker in the handset. Connect ONE of them to your handset speaker. And if you are just using a ringer and no music, connect the other one to a 3W speaker.
The handset has four cables connected from it. Two from the mic and two from the speaker. They are always different colors and never labeled so the only way to figure out which ones go to the mic and which ones go to the speaker is through trial and error. What I did was simply play a track from the shield and then plugged in different combinations until I found the two for the handset speaker.
Note that I did NOT use the mic for my project, but as you probably know, the MP3 Maker Shield has a mic input! So much hacking potential right there :)