The Phone Gangsters

Always have a fake name ready, in case the person called askes for one. Make a funny name that fits in with your voice and character.
Use a funny voice, one you've practiced and tested with your friends.
Don't laugh!!! A single giggle can give it away and totally ruin a good call.
Use sound effects to make the call funnier. You can get free sound effects at www.prankcallsunlimited.com for free.
Don't always call the same person. If you call the same person over and over, the calls probably will get stale (and the person will get mad!)
Make the call sound real, if it sounds like an actual call, the person won't even know it's a prank call. Some people may even get a laugh out of it!