T-Mobile One customers can add an extra line for free for a limited time.

The promotion is for current One customers and does not apply to One Plus customers.

The promotion is in response to a recent study that found T-Mobile’s One plans to be the most valuable in the mobile industry.

Hot off the heels of today’s OnePlus 6T launch, T-Mobile announced it will offer existing customers on its base One unlimited plan a free line of service for a limited time.

Keep in mind that the free line is available to existing customers who have a voice line. Also, T-Mobile offers the free line in the form of monthly bill credits. The carrier did not say how long it will offer the promotion to existing customers, though the promotion is in direct response to analytics firm Strategy Analytics’ latest study on the value of T-Mobile One.

According to Strategy Analytics, T-Mobile One customers receive up to $1,550.71 of additive value on top of unlimited talk, text, and data. That value goes up to $5,150.59 for a family of four. This is thanks to the added benefits T-Mobile One boasts, such as a free Netflix subscription, free Gogo in-flight Wi-Fi service, T-Mobile Tuesdays, and more.

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It also helps that T-Mobile One includes texting and data use while abroad in over 210 countries and destinations, mobile hotspot at 3G speeds, up to 5GB of LTE in Mexico and Canada, and unlimited streaming from services which will not go against your data usage limits. As a nice bonus, the monthly price is a flat rate which includes taxes and fees.

On the flip side, video streaming is capped at 480p resolution when on a data connection, tethering is limited to 3G speeds, and your speeds get throttled to 2G once you use up 26GB of 4G LTE. There is also a One Plus plan which alleviates some of these caveats, but not all of them.

As previously mentioned, T-Mobile also launched the OnePlus 6T today. The carrier offers 50 percent off OnePlus’ latest flagship in the form of 24-month bill credits. However, it looks like the T-Mobile version of the OnePlus 6T might lag behind the unlocked version when it comes to updates.

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