Need: answering machine
Willing victims (your friends who call).
An optional device would be voicemail although for best enjoyment using the answering machine to screen the calls give the best opportunity to hear your caller's reaction.
Step 1: Revise Your Normal Greeting
Simply change your message for your to be "Hello...Hello...Speak up I can't hear you..."
You can add, after a pause or two, "Oh, that's right - this is just a recording, please leave a message - April Fools!".
Your callers will think you're really talking to them, may get mad when they realize they've been duped, and may leave you a few choice words of there own!
As I said, this is best when you can use your answering machine to screen the calls and listen (and maybe talk to) your victims.
It also helps if you can set the machine to pick up right away instead of waiting for 3-4 rings.
This isn't very original, but it's quick and harmless (you might not get some messages).
Have fun and let's be safe out there!
Step 2: Reset Your Greeting to Normal
After April 1st, you need to go back and set you greeting to it's normal, boring message. Systems can vary, but simply re-recording the standard greeting will work. On mine, there are greetings that can be activated for a specific period of time (such as a day). YMMV.
Have fun (and don't get fooled yourself)!