"Customers tell me repeatedly, I love what you're doing, I wish I could switch, but I'm handcuffed," Mr. Legere said.
Verizon's and AT&T's high-speed networks also cover more places than those of T-Mobile and Sprint, the No. 3 carrier. But cracks have started to appear in Verizon's network, long the envy of the industry.
The increased demand for data by consumers has led to reports of slower service in some major metro areas.
Although the turnover rate has been relatively low, many consumers have made the carriers a regular source of derision. In a business conference last year, Daniel Hesse, chief executive of Sprint, cited a survey that found that the reputation of wireless carriers had dropped to the lowest level among any major industry.
"Even the cable and oil industries rated higher than we do," he lamented then.
Mr. Legere of T-Mobile has said that he is on a mission to change that.
Last year, the company eliminated the charges that customers normally paid to use their phone numbers and data services in a foreign country, called roaming fees. T-Mobile also shifted from requiring customers to sign two-year contracts, and it offered an option for customers to upgrade to a new phone after as little as six months, instead of waiting two years.
For T-Mobile's termination fee program, each person can receive up to $650 in credit. To get the customer credit, T-Mobile customers must trade in their own smartphone for up to $300 in credit, depending on the phone and its condition. Then they will receive $350 for each line that incurred a termination fee.
(Read more: T-Mobile CEO shrugs off AT&T's bid to swipe customers)
Customers will have to show proof that they left a carrier and had to pay an early termination fee. Customers of AT&T, Sprint and Verizon Wireless are eligible for the credit.
Jan Dawson, a telecom analyst for Jackdaw Research, said the move was clearly removing one of the big barriers for people to switch to T-Mobile.
"It's a pretty low-risk thing to do — switch to T-Mobile, get a bunch of money for your devices, and then switch back if you don't like it," he said.
"It's a smart move and one that will likely perpetuate T-Mobile's momentum for another few months."
Some analysts say they think T-Mobile is trying to quickly gain customers to set itself up for a sale. Rumors have swirled that Sprint is interested in acquiring T-Mobile, which raises questions about whether T-Mobile's offerings could be taken away under a new owner.
During an interview session with journalists at T-Mobile's conference, Mr. Legere did not dismiss the rumor of such a bid and said, "What I can tell you is the T-Mobile Un-carrier business — the people, the attitude and the brand — is here to stay."