These are done by companies purchasing 800 numbers that are similar to common ones used like 1-800-call-att. They hope that you will misdial the number and generally charge 2-3 times the ammount you expected to pay for the call. In addition they commonly sound exactly like the company you were trying to use and don't identify themselves as being a different company.
More information on this and other common frauds can be found at: www.fcc.gov
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