This actually came into effect yesterday, March 6, offering a third line of service at no extra charge... in certain conditions. If it sounds familiar, that's probably because this same promotion ran for a "limited time" a few months back. There's no word on an expiration date this time around, so you may want to hurry and try to meet the "Un-carrier's" requirements if you like free money.
As far as existing customers are concerned, only those with a single paid voice line on a "qualified" T-Mobile ONE plan with AutoPay seem to be eligible for the third line free after paying for a second line of service. Unfortunately, there's a long list of exclusions, including Unlimited 55+ accounts, 2 lines for $100 plans, T-Mobile ONE 9+ lines, Amplified, Military, and Essentials plans.
Meanwhile, all new customers have to do is activate a total of 3 "qualifying" voice lines on a "qualifying" T-Mobile ONE plan during the (unspecified) "promo period." The same restrictions also apply here, and it's important to point out your savings will be offered in the form of monthly bill credits. You need to keep in mind a few other things as well, like a limit of one redemption per account and mandatory retention for one year of the "same number of lines that are on your account when the promotion starts."
In other words, if you cancel any line on your account, you're giving up your free credit. Even after the first 12 months, you have to maintain two paid T-Mobile ONE voice lines to continue receiving your money back on a third line. That's a lot of hoops and strings attached, but at the end of the day, it's also quite a bit of money saved.