T-Mobile will give you a free Motorola Moto E6 if you add a new line of service1
The Motorola Moto E6 is free at T-Mobile only if you buy it on a monthly payment plan ($6.25 per month for 24 months) and add a new line of service. After 24 months, you'll receive a total of $150 back via monthly bill credits.
T-Mobile's Moto E6 (pictured above) isn't different from the E6 that's available at Verizon. This means it's got a not-too-modern design, but at least its specs aren't too terrible. The handset runs Android 9 Pie, sports a 5.5-inch display with 720 x 1440 pixels, and is powered by an octa-core Qualcomm Snapdragon 435 processor. Other features include splash resistance, a 13 MP rear camera, 16 GB of expandable storage space, and a 3000 mAh battery (according to Motorola, this battery should last "more than a day" on a single charge).
While the Moto E6 is not bad for a phone that can be had for free, T-Mobile has a few other handsets that are offered at no cost when you add a new line. Included here is the Motorola Moto G7 Power, which is superior to the E6 in pretty much every regard, so it's obviously a better choice.
The Motorola Moto E6 is also free at Metro by T-Mobile (T-Mobile's prepaid brand). However, in this case, you can get the smartphone for $0 only if you switch from another carrier.