T-Mobile, Add Free Line with Unlimited Talk/Text/Data
T-Mobile is offering its customers new lines for free. If you already have a T-Mobile account with at least two voice lines, you can add another line for free. You need to have one of these plans to qualify: Magenta, Essentials, ONE, or Simple Choice. You can only add one free line through this promotion.
The promotion was sent out by email, but should be available to everyone. It was offered to me when I called in also.
Sprint also has a similar promotion.
Via monthly bill credits. Limited time offer; subject to change. Free new line for accounts with 2 to 11 voice lines; plus taxes & fees for accounts currently paying for a T‑Mobile wireless line with additional taxes & fees. Qualifying credit and Magenta, Essentials, ONE, or Simple Choice plan required. Credits may take up to 2 bill cycles; credits will stop if you cancel any lines. Limit 1 offer per account.