It could have been a friend playing a prank as well. Like when you leave a voice mail. If you listen to the options, you can leave a call back number, this is any number you want to list.
Sometimes, when a person blocks their number before calling, they are asked to leave a number if the person wants to call them back, again, it doesn't have to be the same number they are calling from.
So when the receiver goes to try and retrieve the number that was listed as blocked/restricted/unavail... they are given the number the caller left.
Sounds like a prank amoung friends happened Especially since you can factually check your usage online, in store, or over the phone.
All of the answers provided can give you sorm sort of reasoning for things like this happening. But their are programs out there that will allow a person to do what is considered Caller ID spoofing, where they are able to make calls from a phone but have the outgoing caller id show someone elses number. This is normally done from landline phones and telemarketers. Telemarketers use it so that people can't call back into their outbound only call centers. But you have to feel for the person who is getting the angry calls with people screaming at him to stop making the nuisance calls.
