I'm attempting to replace the thermostat in my hallway, inside it had a series of numbers & letters T99BV303J do I have to get this ATT product or would any other replace the unit.

Those numbers and letters describe connections. Tell me you didn't just remove the wires without noting what wires were connect to these letters and numbers.

Didn't touch anything, tested the wiring no juice anymore, have to go down and flip the switch.

Okay, then write down the colors of the conductors that are connected to those letters or take an up close picture of the connections before removing any wiring.