My question is more about how devices are locked. How can I tell what

My device that I bought as an unlocked phone almost 2 years ago,all of a sudden locked yesterday. I started a T-Mobile home Internet account and got 1 mobile line with the bring your own device option. As soon as the sim card was put into the phone it immediately worked just fine on their network and with the Wi-Fi. It's been 2 weeks working fine,then yesterday boom ,it says it's locked. We went thru all the resetting procedures and all that while on the phone with T-Mobile,and at the end of it ,it's still locked and T-Mobile doesn't seem to have a reason or answer. How can I find out who locked my device?