If you just want a voice changer check this out http://skypefx.codeplex.com/ or just google Skype voice changer.
If you want to make your calls very hard to trace then:
Get a private VPN service then make a fake email account, setup a Skype account and buy a Skype card with cash from somewhere (Target, WalMart etc..). I'd say that's untracable enough.
The person you call can see the Skype number and report it to the police and if you did something that would be a felony like a bomb threat or something. They would subpoena Skype with the time of the call and the number. Skype would respond with the incoming IP address of the call then the police would have to go to the ISP (the VPN provider in this case) and ask who had that IP address at that time. The VPN provider will probably say "I don't know" since they're not supposed to keep logs, but sometimes they do and they may respond with you IP address. Then the police would have to subpoena the ISP and find out who had that IP address at that time to get your address. Then they would have to get a warrant for your arrest and get it to your local PD who would have to be willing to go arrest you and the investigating PD would have to be willing to pay to have you extradited, which they probably will only do if you have done something bad enough that you won't be able to post bail.
Now that's just my take on it, this isn't legal advice and I'm sure they want you bad enough they will find you. If you're just going to do some prank calls that should be good enough.