It looks like T-Mobile has an upcoming Buy One Get One (BOGO) offer that it will release on Friday, December 17th. 

Details of the BOGO offer were first shared on Reddit. It was then picked up by news outlets and confirmed by The T-Mo Report through internal documents. According to the report, T-Mobile will be offering BOGO lines on its discounted rate plans, including First Responder, 55+, and Military rate plans. 

In order to get the offer, however, you will need to have an active T-Mo account with at least three months of service. The promotion you get will also be determined by the number of lines you already have on your account. This means that customers on an eligible plan with a single line will be getting a Third Line Free promotion. Meanwhile, customers who have two or more lines will get a BOGO offer. 

You can check if your plan is eligible for the offer through the SOC code on Reddit. The promotion will be available starting Friday, December 17th. There has not been any announcement of an end date yet. 

Source: The T-Mo Report