*67 before the number you dial. I assume Wal mart would have a voice changer, of course you could just google it.
Btw, that account you sold me got recovered. I'm not accusing you or anybody for that matter. But I wouldn't suggest trading with whoever you bought it from again.
VOIP is the best thing to use for prank calling.
If you do not want to pay for Skype or VOIP Zoom, just download iCall, its free and works decently.

