T-Mobile offers a second line for free when you activate a phone line1
First off, T-Mobile customers with at least 2 qualifying voice lines on a qualifying T-Mobile ONE, T-Mobile One Plus Family, or Simple Choice plan can add a paid voice line and get an additional voice line for free, after monthly bill credits.
As such, the free line will be redeemed via bill credits and will be applied monthly to the line activated during the promotional period. The deal requires all lines to be activated or ordered during the promotional period. Also, paid lines must be active to continue receiving bill credits.
Keep in mind that only qualifying voice lines may be added for the third paid line or the free line. A maximum of 1 free line may be added per account during the promotion.
There's one other requirement that isn't mentioned on T-Mobile's website, but it's been confirmed by an employee on Reddit, TmoNews reports. It looks like you can't cancel your existing lines and then add two lines to benefit from the deal.
Once again, this deal is only for current customers on qualifying rate plans, not for new customers. If you cancel the line that's receiving the discount or any of your lines within 12 months or migrate to an ineligible plan, you'll lose the monthly bill credit for the free line.