T-Mobile is currently running an offer for a free line. 

Beginning March 17, T-Mo customers with a minimum of two paid voice lines under an eligible plan can get a free voice line via monthly bill credits. The best part about this is that the line will remain free for life as long as you continue being eligible for it. 

To maintain eligibility, you will need to keep the same lines you are using for a year. After that, you need to maintain two paid voice lines to continue enjoying the free line.

This information comes from a leaked internal document that coincides with the offer they made back in 2020. 

The eligible plan SOCs can be seen on this screenshot:

Source: 1