T-Mobile is getting serious about competing with Verizon and AT&T, both of whom are capturing record numbers of iPhone and Android smartphone customers. The company announced an “unprecedented” deal today: zero dollars down for all phones and tablets.

“The number of reasons not to switch to T-Mobile this summer is ZERO,” CEO John Legere said in a statement.

Perhaps, but you’ll still end up paying.

The promotion, which starts tomorrow, offers all T-Mobile devices for zero dollars down — that’s true. iPhones, including the iPhone 5, the latest Samsung Galaxy smartphones, Nokias, BlackBerrys, and various tablets are all available for precisely zero dollars down. Which sounds like a great deal.

The catch is that the cost of the device will simply be added to your monthly bill.

A selection of T-Mobile devices available at $0 down, with monthly pricing

Above: A selection of T-Mobile devices available at $0 down, with monthly pricing

Image Credit: T-Mobile

So that Samsung Galaxy S4 will cost you $25 a month for 24 months, instead of the standard T-Mobile instant purchase price of $629, or instead of the typical T-Mobile downpayment of $149 plus 24 monthly payments of $20/month. You still save some money, because with the new promotion, the device is $600, versus $629 for instant purchase or the downpayment plan.

And, since there’s no interest — and no contract — it’s still a good deal.  (The monthly payment is to a financing agency, not T-Mobile, so you could theoretically leave at any time.)

The promotion is “time-limited.” When VentureBeat editor Devindra Hardawar asked T-Mobile what that meant, precisely, T-Mobile representatives were not incredibly clear:

“Good question. This is a summer time promotion for a limited time.”

In other words, it could be cancelled at any moment. Get it while it’s available.