Spoof and trick the Caller ID
I talked about a service called Phone Gangster, located at http://www.PhoneGangster.com on the how to call people anonymously page. They offer some of the best anonymous phone cards around and you get to pick any caller id you want. PhoneGangster even offers a free 5 minute card for you to check out the service before buying. I would definetely recommend this as a neccesity to be anonymous.
Here is a screen shot of their online web panel interface:
