How Reliable is Digits? Is this normal....

I have 1 phone with a Tmobile plan and using Digits app on my 2nd phone which is a Sprint plan with Tmobile SIM card… from what I understand it shouldn't matter what carrier you're using as long as you have wifi or cell signal the calls & texts should come through to the app.  I HAVE noticed that verification texts (from banks, password resets, etc) will NOT carry over to the Digits app and will only show up on the device with my Tmobile plan/SIM…. what's strange is that I don't receive texts from certain personal numbers (not sure what carrier) but I do know of one instance where a Tmobile customer had been trying to reach me via text several times per day for 4 days in a row and also tried calling me and none of the calls or texts came through to the Digits app.  I would have thought that if any texts & calls were relaible it would at least be from Tmobile numbers. I have experienced issues in the past using VOIP apps where Verizon customers never received my texts but I wasn't expecting a Tmobile service to not function between it's own customers?