This is a version of the automatic debit scam as described by the FTC here http://www.ftc.gov/bcp/edu/pubs/consumer/telemarketing/tel06.shtm  It's frequently mysterious as to how they got your account info in the first place. They might be fraudulently charging accounts at random, or you might have provided your account for a free something like $-off coupon or a shopping card (just pay for shipping), or provided your account when you applied for a payday loan, and buried in the fine print is authorization to auto-charge your account every month with 3rd-party, perhaps difficult-to-cancel subscriptions. The methods used to get you to "agree" to the fine print is endless and they come up with new schemes every day.
By law, all 3rd-party phone billers have to include phone numbers with their charges. If you don't see a number on your bill, call your phone company, tell them you need the biller's number and they should be able to retrieve the number for you from their records. To stop the billing, call the biller to kill the subscription and get your money back and if that fails, tell your card issuer or bank that fraud has occurred and have them refund the money and replace the account - this is important, the account must be replaced because criminals have it and they can charge again or sell account info to other criminals. Report to the Internet Crime Complaint Center http://www.ic3.gov/complaint/default.aspx , the Federal Communications Commission http://esupport.fcc.gov/complaints.htm , the Federal Trade Commission https://www.ftccomplaintassistant.gov/ and your state AG, as appropriate.

my bank was charged 99.95 on April 1, 2011 and I don't know what the charge was for.