The rollover data wave, started by T-Mobile, has washed over AT&T's postpaid service and is trickling down to the company's prepaid GoPhone service on May 15.

Like rollover minutes, schemes in which unused calling allotment carry over into the next month, rollover data plans enable users to make better use of their monthly data, which is especially valuable on prepaid calling plans where allowances are generally slimmer.

The new rollover data will be available to new and existing customers who subscribe to a $45 or $60 plan. Existing customers will be able to roll over data at the end of their current plans, as long as the monthly renewal date comes after May 15.

"For example: A customer signs up for an AT&T GoPhone $60 plan on May 16, 2015, and he or she uses only 2.5 GB of the 4GB high-speed data available with the monthly plan," says AT&T. "When the customer renews their $60 plan on June 16, 2015, they will have 4GB high-speed plan data +1.5GB high-speed Rollover Data to use over the next 30 days."

To keep the data banking in check, there are a few restrictions on it. Rollover data only carries over for one 30-day period and the cached data is used only after the monthly allotment runs dry.

So if a user rations data and stays within his or her allotment for two consecutive months, the first month's data savings will disappear by the time the third month arrives.

T-Mobile introduced rollover data back in December of last year, drawing inspiration from the roughly $1.5 billion in overages that wireless carriers collected in 2013.

T-Mobile CEO John Legere likened disappearing data to gas stations siphoning unused gas out of a person's car at the end of every month. The wireless industry is worse than that, he said.

"Americans have been gamed by the carriers into buying huge data plans -- all to avoid getting screwed with overage penalties," said Legere. "Only to find out they bought more than they need, which is then confiscated by the carrier. For the consumer it's lose, lose."

While the self-styled "Un-Carrier" championed rollover data, it also places restrictions on the life of cached data. As is the case with AT&T's rollover data, T-Mobile's banked data is used only after the customer's standard allotment has been depleted.