T-Mobile has just announced that they are doing a new promotion for those looking to add a third line to their account. If you are already on their current promotion that gets you two lines for $100 on T-Mobile ONE, you can get a third line for free, and this is via bill credits. T-Mobile notes that users with two voice lines can get this third line for free and use it for anything they want. That could be a smartphone, a tablet or even a wearable like the LG Watch Sport or the Samsung Gear S3 Frontier. Keep in mind that the 2 for $100 offer is only valid if you have autopay on your account, if not, it'll be $105 per month. However, if you don't use autopay you can still get this third line for free. It'll just cost a bit more.
Wireless carriers are currently looking for ways to bring in more customers and add more connections for the customers that they already have, which is why you've been seeing companies - namely T-Mobile and Sprint - offering additional lines for free. Over the holidays, T-Mobile offered up free lines for those on their Simple Choice and T-Mobile ONE plans, and now they are doing it again. It's actually a good way to get customers to sign up for another line, as they can get another smartphone on that plan and get it for free. Essentially being $33.33 per month per line when you have three lines for $100.
T-Mobile mentions in their press release that while this promotion is only valid for a limited time, the third line will be free as long as you are on an eligible plan and keep current on your bill (meaning you don't let it go past due, or default on payments). That's compared to Sprint's promo which raises the prices of those three free lines to their regular price, in March 2018. You can get this free line online at My T-Mobile or by going to your local T-Mobile store and getting it setup in person. T-Mobile has plenty of products that you can use on that third line.