Today’s multitalented cell phones can easily seduce you into texting, talking or downloading more than you planned. The result can be hundreds of dollars in unexpected penalty fees.

Some 30 million Americans have experienced cellular bill shock, a surprise spike not related to a change in the service plan, according to Consumer Reports. The projection is based on a Federal Communications Commission survey of 3,005 adults conducted last spring. More than a third of that group got burned for at least $50 a month. CR thinks the usual overage charges might still be higher. Based on a consumer billing data from Validas, a company that provides wireless bill analysis and optimization services, CR estimates the median overage charge to be about $85.

Overage fines often come from using more voice minutes than the monthly plan allows, at a cost of 25 to 45 cents per minute — at least four times the regular per-minute plan rate. But big texting charges are also a potential problem. Among ConsumerReports.org subscribers surveyed by the Consumer Reports National Research Center last year, about 12 percent of texters who didn’t have an unlimited texting plan were surprised by higher-than-expected messaging charges.

An emerging threat is exceeding limits for downloading data to your smart phone when you access the Internet, send e-mail with or without attachments, post photos to Facebook or engage in other high-bandwidth activities. The industry usually charges one monthly fee for unlimited data. However, in June AT&T began selling monthly data service for its popular iPhone and other smart phones in rations of 200MB for $15 and 2GB for $25.

AT&T’s new pricing might save most customers money at current rates. And at least one other carrier, Verizon, says it might eventually follow suit. That makes it all the more important now to track your data usage, especially since new apps constantly encourage higher consumption.

How to protect yourself

• Ask for a break. Request an exception or discount. Carriers say they work to resolve overage problems on a case-by-case basis. One CR staffer knocked $175, or 68 percent, off his $255 overage bill and saved an additional $23 in taxes, fees and surcharges.

• Monitor your usage. Depending on the phone and carrier, you can keep track by checking settings, tapping the usage-information codes on your cell phone, or registering with your carrier for online access to your cell-phone account. Try to check your balance at the middle of and three-quarters through each billing cycle, especially in months with unusual use.

• Sign up for an overage alert. A few carriers offer them. For smart phones you can get the free Cell Minute Tracker app from Pageonce, a California software company. It alerts you when you’re nearing the limits of your voice, message and data allotments and lets you know when you go over them.

• Adjust your plan as needed. If you’re bumping up against your limit for voice calls, switch to a higher-minute plan. To find a contract plan that better fits your needs, analyze your current use and the available deals. The website BillShrink searches scores of plans from the four largest carriers free, and Validas searches six service providers for $5; neither searches prepaid carriers.

• Monitor and adjust your message and data services, too, if they’re metered. Users of iPhones tend to consume more data per month than BlackBerry and other smart-phone users. If you’re with AT&T, the 200MB-per-month package should meet your needs, but heed the alerts.

• Consider prepaid. Available from the major carriers and from prepaid specialists such as Virgin Mobile, prepaid, pay-as-you-go plans have no overage charges. Instead, you buy more minutes at the regular rate when your account balance runs low or runs out. Or consider a monthly plan with unlimited minutes from a prepaid carrier, which can cost less than such plans from contract carriers.