@ tzviR update?
I don't use it in my current set up but I used to have a T-Mobile sim in a flip phone and was able to make and receive calls and text at the number through the Digits app. I have a different set up now but it should still work the same. Try it out and let us now.
Because this comes up somewhat often I'm going to explain my set up in case it will help someone. It is a little different than what many people want as I rarely carry the smart phone with me but this is what works for me.
I have an LG classic flip with US mobile service. This is my main phone and the number that everyone has. Most calls and texts I make are from this phone.
Very useful and almost necessary for anyone that uses a flip phone is google voice. If I ever have to write up a long text I'll just type it up on my computer and send to myself through voice and forward it to whomever. Also if anyone ever sends me a link or a picture I need to see more clearly I just forward it to my google voice and look at it on my computer.
I also have an iPhone with T-Mobile filtered through TAG with MB Smart. It stays in my car and I have it for Carplay/maps and I'll also take it with me when I travel, great for mobile boarding passes and on the go traveling needs. Almost no one has that number. It was too confusing for people to have a second number for me that I didn't always have on me. I have the caller ID blocked and sometimes if I'm on a call with someone and want to switch it to the car system I'll tell them that I'm calling them from an unknown number and they should pick up.
Personally this setup works well for me. I find that only carrying a flip phone with me comes with many benefits and removes so many distractions but having the iphone in the car minimizes the inconveniences of having a flip phone.