(MONEY Magazine) – New York City's Central Park may provide the illusion of isolation. But that doesn't explain why, when he reviewed his bill last month, Erik Buck found he'd been charged 64[cents] a minute for "roaming" in the epicenter of the most densely populated area in America and less than a mile from his home.

Turns out, erroneous roaming fees are a persistent problem with most cell-phone carriers--and especially with Buck's Sprint PCS, says Yankee Group telecom analyst David Bishop. (A Sprint spokesman called the case an anomaly possibly caused by a defective handset.) Cell phones have grown so popular that carriers have not been able to build new signal towers and upgrade old ones fast enough, resulting in spotty coverage even in large metropolitan areas.

So what can you do about it? First, check your prospective carrier's coverage area carefully. Two independent sites provide better detail than the carriers themselves: www.decide.com and www.point.com. Second, before you place a call, check your phone's display screen for an indication that you've left the digital network--an R usually indicates roaming. And always scan your bill for errors and dispute any strange charges--most service reps, says Bishop, will gladly credit your account.

--GRACE JIDOUN