Why does my sim card have no signal 24/7?

[Android Central Question]

I just bought an unlocked Verizon phone a few months ago and I put my old tmobile sim card, that I had in my previous phone in it. And I've had no reception 90% of the time. I've changed the apn settings, restarted, went through all the settings and searched all over the internet and I dont know what it is. The icon is a triangle with an x above it. In the sim settings it says its unconnected or not showing up, but sometimes I'm on wifi and it shows up in the settings but as soon as wifi is gone the signal strength lowers and drops down to the negatives and I lose all connection, if you could help that would be greatly appreciated, thanks!

 

[B. Diddy]

Welcome to Android Central! Which phone? Are you certain it supports T-Mobile's network bands?