Alerting consumers to data limits is particularly relevant with the explosive growth of the iPad and other tablets, which can consume immense amounts of data in downloading music and books, and streaming movies. The F.C.C. has said that the popularity of tablets and the accompanying growth in data use is contributing to overcrowding of the airwaves, with wireless companies finding that they may eventually not be able to accommodate the demand for downloading.

A 2010 study by the F.C.C. found that one in six mobile device users had experienced bill shock, with 23 percent of those users facing unexpected charges of $100 or more. A separate F.C.C. report noted that 20 percent of the bill shock complaints it received during the first half of 2010 were for $1,000 or more in overage charges. Expensive charges can also be incurred for roaming, when a user travels out of a company’s defined area of coverage or, as often occurs, when traveling overseas.

Even so-called unlimited data plans often have a cap limiting downloads each month to a certain number of megabytes — a technical measure that, unlike a number of calls or minutes, cannot easily be tracked by the uninitiated. Last October, the F.C.C. highlighted the case of a 66-year-old retiree in Dover, Mass., who received an $18,000 bill after a promotional no-limit data plan expired without warning.

Alexander Cullison found out the hard way what can happen when a family member is unaware of usage limits and accounting. Mr. Cullison, a retired resident of Fairfax, Va., received a $400 bill one month recently after his son, whose plan had a monthly limit of 250 text messages, sent and received about 2,000 in one billing period. It was then that Mr. Cullison learned that his wireless company counted each message sent and received as separate items, causing them to build up at least twice as fast as expected.

“This is a good resolution,” Mr. Cullison said, “as long as they advise you that you are going to go over your limit before it actually happens.” Under the agreement, carriers will provide alerts when consumers approach and then exceed their limits on voice, data or texting. In addition, users will receive an alert when their phone links to a cellular system in a foreign country. Some carriers already provide similar alerts.

Companies have the option to deliver alerts by text or voice, but they must be free and automatic. Consumers can opt out of the service if they choose. At least two of the four types of alerts must be started by carriers within 12 months, and all alerts must begin within 18 months.

The companies also agreed to publicize tools for consumers to monitor their own usage. The F.C.C. has teamed with the nonprofit Consumers Union to track companies’ compliance.

“Consumers have been telling us about ‘bill shock’ for a long time, and we’ve been pushing for reforms to crack down on the problem,” said Parul P. Desai, policy counsel for Consumers Union. “Ultimately, this is about helping people protect their pocketbooks, so we applaud the F.C.C. and the industry for this effort to do right by consumers.”