Make sure you can call from an anonymous id. Make a fake Gmail id. Gmail allows you to call to USA or Canada from their website. You can sign in to that website and dial the number that you want to play the joke with.
You can call through a fake Skype id as well but in order to call your Skype to a cell phone, you need to have credit in your Skype id. You can make free calls from Skype to Skype id only but not to a phone number. However, Skype can be installed on phones as well and you can make a prank call in that fashion.
Make sure that doing a prank with the targeted person will not hurt you later. You better know that person otherwise you might fall in a trap of some drug dealer who will track you down and play a prank with your life that you will never ever forget.
You need to have a fake name and everything related to a personality that you want to be. You can’t make a single mistake and unleash your true colours because you can get into trouble as well let alone busted.
If the other person knows you personally, change your voice a bit so that they don’t know who you are otherwise you will get caught very quickly and your whole plan will flush down the gutter.
Write pointers that you will say during the call and when you are ready dial the number either from Gmail or Skype.