Cell phone numbers are being released to telemarketing companies. You could start to receive sales calls and these calls will go towards your monthly usage.
To prevent this, call the following number from your cell phone: 888-382-1222.
It is the National DO NOT CALL list. It blocks your number for five (5) years and you must call from the cell phone number you want to have blocked. You cannot call from a different phone number.
You can also simply register online via the following link:
https://www.donotcall.gov/default.aspx
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http://www.anysitesupport.com/how-do-i-block-my-phone-number-from-telemarketers/