T-Mobile USA is giving free Netflix subscriptions to customers who buy at least two lines of T-Mobile One service.

"Through an exclusive new partnership with Netflix, T-Mobile One family plans will now come with Netflix—at absolutely no extra charge... anyone with two or more qualifying T-Mobile One lines can get Netflix on us," T-Mobile announced today.

T-Mobile One is the carrier's unlimited voice, data, and text plan, and it costs $120 a month for two lines ($60 for each line). The per-line charges go down as you buy more lines. The Netflix promotion will begin on Tuesday, September 12. Starting on that day, T-Mobile customers with at least two lines can activate their free Netflix subscription online, in T-Mobile stores, or by calling T-Mobile customer service.

"[I]f you already have a Netflix subscription, T-Mobile will cover the cost of a standard subscription for you—meaning you’ll save nearly $120 every year," the carrier said.

Works on any device, not just T-mobile phones

The subscriptions will work just like video plans purchased directly from Netflix, so you can watch on any Netflix-compatible device. Viewing is not limited to T-Mobile-connected cellular devices, T-Mobile told Ars.

T-Mobile customers will get the standard Netflix plan that normally costs $9.99 a month, which includes HD video and allows viewing on two screens simultaneously.

Netflix also sells a premium subscription for $11.99, which adds 4K video and allows viewing on four screens at once. T-Mobile said that customers can upgrade to Netflix's premium subscription by paying the $2-per-month difference to T-Mobile.

It's not clear how long the promotion will be available. But for people who sign up in time, the Netflix subscription will be "good for as long as they are T-Mobile customers," T-Mobile told us. You can find more details here.

Carriers are increasingly bundling video with mobile phone subscriptions, in some cases as free add-ons and in others as higher-priced bundles. AT&T has been bundling DirecTV with its mobile service and exempting DirecTV from its mobile data caps. AT&T has also bundled HBO with unlimited mobile plans. Verizon Wireless has made its own Go90 service a free add-on and exempted it from data caps.

T-Mobile started exempting third-party video services such as Netflix from data caps in late 2015 (while downgrading video quality), but the company has moved away from data caps and overage fees since announcing the T-Mobile One plan. T-Mobile has also been giving customers free MLB streaming subscriptions.