Sprint (NYSE: S) has a new limited-time offer that gives customers a cheaper alternative to T-Mobile's (NASDAQ: TMUS) unlimited plans.

The deal, which offers customers five cellphone lines for $90 a month (plus taxes and fees) dramatically undercuts its rival's price, albeit only for a little over a year. It's a bold move by the No. 4 carrier in its efforts to overtake its closest competitor in the battle for wireless customers.

What is Sprint offering?

For a limited-time Sprint is offering one line with unlimited data, talk, and text for $50 while adding a second costs $40 more. But beyond that, customers can add up to three more lines for free. All the deals require autopay, and Sprint, like T-Mobile, reserves the right to slow down data for the heaviest users during peak periods of network use.

The promotional pricing lasts through March 31, 2018. After it expires, customers will pay $60 for their first line, while the second stays at $40, and lines three through five will cost $30 each.

"Sprint understands the value of unlimited data to our customers," said  Chief Marketing Officer Roger Solé in a press release. "Customers do more with their phones every day and they want to use them without limits."

What does T-Mobile offer?

It's worth noting that what Solé said sounds an awful lot like the message T-Mobile CEO John Legere regulrly delivers. His company's unlimited plans also require autopay enrollment, but unlike those of its rivals, T-Mobile's are all-inclusive prices, including all fees and taxes.

The company charges $70 for one line, $120 for two ($60 each), $140 for three ($46.67 each), and $160 for four ($40 each). That's more than Sprint's new promotional deal, but -- for families with three or four lines -- it's better deal than Sprint's normal pricing.

Aggressively courting customers

This is a very aggressive offer from Sprint. It should appeal to people looking for the absolute cheapest price who are willing to deal with perhaps having to change carriers in a little over a year when the deal ends. Even for those not willing to do, that this offer could be tempting because even after it ends, Sprint won't be that much more expensive than T-Mobile (though the bill will be higher than the advertised rate because T-Mobile's prices include taxes and fees while Sprint's does not).

10 stocks we like better than Sprint
When investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.*

David and Tom just revealed what they believe are the 10 best stocks for investors to buy right now... and Sprint wasn't one of them! That's right -- they think these 10 stocks are even better buys.

Click here to learn about these picks!

*Stock Advisor returns as of February 6, 2017

Daniel Kline has no position in any stocks mentioned.