The catch is TextNow’s service requires use of its app rather than the native Phone app and is supported by advertising. Last I played with it, I found the ads annoying. Still, if one is in the market for an emergency backup service on T-Mobile’s network, free is a tough price point to beat. 
https://www.textnow.com/blog/introducing-free-essential-data
I’ve used TextNow for a number of years now. “Essential Data” was added to my account ~6 months ago. I’ve been pretty happy with it, but I don’t use it as my daily driver. I have the app installed on multiple phones and use it as a forwarding destination for several VOIP numbers. You can also send/receive calls/texts from a browser if you wish.
Yup, it was previously added to certain accounts. I don’t know what the criteria for that was. The good news, it’s now available to all including new subs.
Are TextNow numbers still housed on Bandwidth’s network?
My TextNow number currently shows it’s an Onvoy number.