Does tmobile charge any fees for iMessage on Wi-Fi?

Hi, I have an unlocked iPhone 4 and I've had it for a few months now but I've been scared to turn on iMessage in fear that tmobile will charge me fees… Does anyone know if tmobile charges for iMessage when on Wi-Fi? I have my data turned of btw. I'd appreciate it if someone knows.

Over Wi-Fi, T-Mobile should not charge you any fees at all. It's not a network they control.