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Ending the year with another “Un-carrier” move, Bellevue-based T-Mobile US on Tuesday announced a program to allow customers to rollover unused wireless data.

Starting in January, T-Mobile will offer “Data Stash, to new and existing Simple Choice customers who buy a plan with at least a 3 gigabytes of high-speed data for smartphones and 1 gigabyte for tablets.

Initially, each qualified customer will receive 10 GB of free data in their “stash.” Once that free stash is used up, customers will begin building a reserve with any unused monthly data. The data will accumulate and will be available for customers to tap into for a year anytime they go over their data limits.

“It’s your data,” T-Mobile CEO John Legere said Tuesday morning during the announcement, which was a live video stream interview with David Pogue of Yahoo. “What you don’t use, you won’t lose.”

T-Mobile has been touting its Un-carrier offerings since it ended annual contracts in March 2013 to gain market share from the nation’s top three wireless carriers — Verizon Wireless, AT&T and Sprint. Since then the company has announced other moves ranging from free data for life and no international roaming to no data overages for streaming music and test driving the T-Mobile network with an iPhone for a week.

Known for his use of social media to identify problems in the wireless industry and with his company, Legere said he has seen more than 40,000 tweets from wireless customers from all carriers asking about data rollover options, which lead to Tuesday’s Un-carrier 8.0 announcement.

According to Legere, the average T-Mobile customer uses about 3.5 GB of data per month. The Simple Choice phone plans available range from $50 for a 1 GB plan, $60 for a 3 GB, $70 for a 5 GB to $80 for an unlimited plan.

Unlike its competitors, T-Mobile customers who go over their data allotments are not charged. Verizon and AT&T charge $10 and $15 per extra GB, respectively.

With the current set up, Legere said customers are forced to purchase buckets of data that are beyond what they need and at the end of the month, the unused data the customers have paid for disappears. Customers are basically throwing away money, he said.

For example, a T-Mobile customer who buys a 5 GB data plan, but only uses the average 3.5 GB, previously would have lost that unused 1.5 GB at the end of the month. Now it will be carried over to use in a month that the customer decides to possibly stream multiple TV episodes or movies, which takes a lot of data.

Mike Sievert, chief marketing officer for T-Mobile, said Tuesday he thinks data rollover is the biggest promotion since the company eliminated annual contracts last year to get more customers.

And it seems to be working.

Legere said he expects T-Mobile to finish 2014 with a total of 8 million net customer additions for the year, implying the fourth quarter will see about 1.8 million net adds.

In the third quarter, T-Mobile added 2.3 million customers, compared to Verizon, which gained 1.5 million, AT&T, which gained 2 million and Sprint, which gained 484,000.

Since ditching annual contracts, T-Mobile has added 10 million customers — bringing its total at the end of the third quarter to 52.9 million — only 2.1 million customers shy of its closest rival — Sprint.

Last week, T-Mobile announced an unlimited family plan. The plan starts at $100 for two people and is $40 per additional line. Unlike the un-carrier moves, which are permanent, the unlimited family plan is a temporary offering with no current end date.

T-Mobile stock closed down 68 cents, or 2.69 percent, at $24.57.

Coral Garnick: 206-464-2422 or cgarnick@seattletimes.com. On Twitter @coralgarnick