If you have a Skype account (see: http://www.skype.com), you can use it to call any Address Book contacts.
This topic explains the steps for enabling Skype in Address Book:
If you do not have a Skype account, set up the account prior to using Skype with Address Book.
Click a contact in your Address Book. Next, click the phone number of that contact.
A drop-down box lists Convergence's offerings of click-to-call services.
Click Call using Skype to connect the call with your Address Book contact.
Note - The Skype application opens on your system; it either asks you to enter login credentials or automatically logs you into your account (if Skype retains your login information).