Overage charges may simply reflect a customer’s intentional extra use of a service, suggests Christopher Guttman-McCabe, vice president for regulatory affairs at the C.T.I.A.. “There are months when my water bill or electricity bill goes up significantly,” he said. “That doesn’t mean I’m unhappy with the water or electricity utility.”

I wonder, however, whether cellphone service is like other utilities. There’s no water faucet in my house that I could turn  intentionally or accidentally  that would lead to a $68,505 bill for the month. That’s the amount of the largest bill-shock complaint received by the F.C.C. in the first six months of 2010. (An F.C.C. spokeswoman said that the commission’s staff had investigated and confirmed the details of that particular complaint.)

Mr. Guttman-McCabe says the F.C.C.’s alerts requirement would cost “tens, if not hundreds, of millions of dollars to implement.” But the C.T.I.A.’s own written statement describes an industry that has already made considerable investments in infrastructure that enable real-time alerts about use.

AT&T already provides three alerts to iPad users: when data use has reached 80 percent of the plan’s allocation, again at 90 percent, and once more when the allocation has been reached and overages begin. The C.T.I.A. mentions this example but says Apple iPad users “can” receive alerts rather than saying they “will” receive them. The phrasing implies that users must opt in. In fact, the alerts are automatic as well as free  exactly what the F.C.C. would like to provide all cellphone users.

THE C.T.I.A. cites the SmartAccess program from T-Mobile among exemplars of “alerts and cut-off mechanisms” that carriers provide without being compelled to do so by Washington. It said SmartAccess allows customers to set a spending limit and, if their account charges exceed the limit, service automatically stops until payment is made to reduce the balance.

But consumer protections not mandated by Washington can disappear at any time. For example, a week after the C.T.I.A. praised it in its filing, I couldn’t find SmartAccess among offerings at the T-Mobile Web site. When I asked T-Mobile what had happened, a spokesman said SmartAccess no longer existed. (The next day, a spokeswoman offered T-Mobile’s $4.99-a-month “Family Allowances” program as an equivalent service, but it doesn’t cover data use and comes with a disclaimer that it “is not intended to prevent overages.”)

Currently, most wireless customers must remember to manually check their use tallies. And check again and again. Without the protection of mandated warnings, they risk being the next $68,505 loser.