Right, the easiest way to do this is to use a site called Bluff My Call (bluffmycall.com/), where you can change your number to anybody’s in the world. To use this service more than once you have to pay, but there is the option of a free trial. You will have to provide credit card details for the free trial and it works on the basis of prepaid minutes, like a pay as you go phone. $10 will give you an hour’s worth of call time, will $29.95 will give you unlimited calls in one calendar month.
The caller ID on the person’s phone will reflect whatever you want, and will even show the name registered for the number you chose if it’s saved on the recipient’s phone. Bluff My Call also offers a recording service, so if it is a particularly good prank, you can keep it for many hilarious replays.
Although morally dubious it is legal throughout the U.S. And most other parts of the world (unless you are changing the caller ID to commit a crime or to conduct any illegal activity). Also some states require the consent of both parties in order to record a conversation.
Alternatively if you have an iPhone or Blackberry, there’s a Spoof ID caller application called SpoofApp (spoofapp.com/), though I’m not sure of the security of this service and if you can be traced back. Again as the call goes through this company’s servers you will be required to purchase minutes off them.
The only free way I know of calling someone from someone else’s number is to actually use a different phone. Sometimes the simplest methods are the best.