T-Mobile has announced it will roll out a 'truly unlimited' 4G data plan next month, in a move aimed squarely at its larger rivals Verizon and AT&T, which throttle speeds after a certain limit has been reached. The new plan will cost $20 per month when added to a $49.99 value unlimited talk and text plan (no smartphone subsidies) or $30 per month when added to a $59.99 classic unlimited talk and text plan.

The only catch is that T-Mobile won't let you use your phone as a mobile hotspot without paying a separate fee. Additionally, it should be noted that T-Mobile refers to HSPA+ data service (up to 42Mbps in some markets) when mentioning "4G data" and not  4G LTE. The company isn't saying if users that sign up for the plan will be able to access its upcoming LTE network with the same limitless data access.

Both Verizon and AT&T quit offering unlimited plans a while back, but have plenty of grandfathered users holding onto them for as long as they can. Instead they are pushing shared data plans that offer "buckets" of data for up to ten devices – including laptops and tablets – in the same household. But they are comparatively much pricier: AT&T Mobile Share plans start at $85 per month for 1GB and one smartphone with unlimited text and talk, while Verizon charges $90 for a similar plan.

T-Mobile is not alone in offering unlimited data plans in the US. Sprint, the country's third largest carrier, has been using its unlimited plans to differentiate itself from its competitors for some time, while MetroPCS Communications recently cut the price of its unlimited-data plan from $70 to $55 a month.

It's also worth noting that T-Mobile will continue to sell plans capped at 2GB, 5GB or 10GB of data per month. Unlike Verizon Wireless or AT&T, T-Mobile doesn't charge overage fees for customers who go over those limits, but their speeds are slowed down considerably.