(MONEY Magazine) – Cell-phone connections can be fuzzy; so can their payment plans and bills. About 17% of Americans use cell phones, according to the Cellular Telecommunications Industry Association, paying an average $47.70 a month for the service. If you've got a cell phone or are thinking about buying one, beware of these traps:

--The service plan that customers pick often isn't the most economical one. Service providers typically ask you to choose among several different plans, then sign a contract that locks you in for one to three years. Most customers opt for the plan that charges the lowest monthly base cost (typically $20 to $25), though it comes with the highest per-minute fee (25[cents] to 40[cents]). "Most people wind up using their cell phone far more often than they expect," says Tim O'Neil, a telecommunications analyst at SoundView Financial Group in Stamford, Conn. Ask your provider to let you switch to a more economical plan before your contract expires; many will oblige.

--Free minutes aren't really free. Most plans offer you 60 or so "free minutes" of phone usage in your local area initially. However, Baby Bells charge cellular companies 5[cents] to 12[cents] a minute for access to regular phone lines, which your cell phone taps if you call a regular phone, and most cell companies pass the charges on to you even for those bonus minutes.

--It's tough to know when you're roaming. Almost all companies tack on a roaming fee of up to 90[cents] or so a minute for both incoming and outgoing calls while you're cruising outside your local area--typically, your city and its suburbs. Read the fine print on your contract to find out exactly where your roaming charges begin and end, and keep them in mind every time you call.

--Many providers' bills make a Rothko painting look detailed. "Our bill doesn't list the numbers we called or say which calls racked up roaming charges," says Ann Marie Marcellus, 28, an oboist in Alexandria, Va. who uses Cellular One. "All it tells us is how many minutes we used the phone." If you suspect you've been overbilled, call the provider and ask for a call-by-call accounting--and demand that mistakes be removed before you pay up.

--Jennifer Zajac