The battle for your cell phone business is heating up with T-Mobile's no-contract promotion that includes paying early termination fees for those who dump Sprint, Verizon or AT&T.

The offer targets families as it offers to pay the break-up penalty up to $350 for up to five lines.

How does the offer work? And what's the catch?

When you break a contract with Sprint, Verizon or AT&T, you have to wait for T-Mobile to reimburse you.

"When the early termination fee appears on your bill, you have to turn it in to T-Mobile or reimbursement," Consumer Reports' Mike Gikas said. "So, you actually have to pay that money up front."

You don't keep your old phone, either. You have to buy a new phone for every  line you switch. That can run $200 to $600.

However, T-Mobile will ease that pain with a credit for the phone your turn in, up to $300. That amounts to about $100 for an iPhone 4S and $160 for a Samsung Galaxy, according to T-Mobile's website. The amount depends on the condition of the phone.

As for choosing a plan, T-Mobile promises "all plans come with unlimited talk, text and Web." But, with lower-cost plans, you only get so much data before the speed slows, according to Gikas.

Still, Consumer Reports found there could be reason to switch.

"We've crunched the numbers, " Gikas said. "And in most cases, T-Mobile's rate were cheaper than those from AT&T, Sprint and Verizon."

And, you don't have to sign a contract.