You could end up being charged for a service on your monthly mobile phone bill even though you never asked for it. This practice, called “cramming,” occurs when a provider other than your phone company supplies you with ringtones, sports scores, weather updates, horoscopes and/or other unwanted services, and your phone company adds a charge to your bill, typically several dollars. (With landlines, cramming usually shows up as a line item for “miscellaneous” or “enhanced” services.) The phone company keeps a portion of the revenue. Although three major telephone companies—AT&T, CenturyLink and Verizon—have agreed to end most third-party billing for landlines by the end of this year (and some have already done so), they will continue for wireless and smaller landline providers unless you opt out.

Self-defense: Call your phone company now, and request that all third-party providers be blocked. Review your phone bills every month for surprise charges, often listed as “service charge,” “other fees,” “calling plan” or “membership.”

Also, avoid calling “900” numbers, accepting anonymous collect calls and signing up for contests online via your cell phone, all common methods that vendors use to cram you.

If you do get crammed, call your service provider and demand a refund.