Credit: Mark Lennihan/AP Photo

While AT&T and Verizon move to shared data plans, which allow users to share a bucket of data across multiple devices, T-Mobile has a different approach.

Today, the U.S. carrier announced that it will begin offering unlimited 4G data plans for smartphones starting Sept. 5. The plan will cost a very affordable $20 a month when added to one of T-Mobile's value voice and text plans and $30 when added to a classic voice and texting plan. T-Mobile will offer an unlimited calling and text plan with the unlimited data for $69.99.

T-Mobile's unlimited data plan offers unlimited Web surfing and app usage over T-Mobile's HSPA+ network. There are no data caps or speed limits. The plan is only for smartphones, not tablets, laptops, 4G connection cards or mobile hotspots.

In comparison, AT&T's plans start at $40 a month for just 1GB of data to share across 10 devices and unlimited voice and texting - and every additional phone you add to the plan costs $45 a month. AT&T's highest offering, which includes a cap of 20GB of data, costs $200 a month with unlimited texting and calls.

Since introducing its respective data share plans in July, AT&T and Verizon do not offer unlimited data for new customers. Sprint offers unlimited 4G, texting and talk for a base of $100.

"AT&T and Verizon are in the costly and confusing camp and Sprint's offering is limited since their 4G coverage is limited," T-Mobile's vice president of marketing, Kevin McLaughlin, told ABC News.

Sprint began offering a limited LTE network this summer in Baltimore and other cities. T-Mobile's 4G network is not LTE, though covers much of the country.

It plans to roll out its LTE network in 2013, though it notes that its HSPA+ network is faster than even some LTE networks in parts of the country.

T-Mobile has taken a hit in the last couple of years in a large part because of the fact that it doesn't offer the iPhone. T-Mobile is hopeful that this plan will help pull customers from other networks, even if it doesn't have Apple's popular phone.

"We are trying to serve the customers and there is a lot of need in the market with Verizon and AT&T's new plans," McLaughlin said. "They are frustrated with the fact that they cannot get unlimited data."