If iMessage on your iPhone or iPad is not working please do the following steps. Please also note that iMessage on 3G/4G will only work on your iPhone if you chose the USA Extreme Plan (since the other plans do not include data):
1. On your iPhone go to Settings, Messages and turn iMessage OFF and then back ON.
2. Tap "Send & Receive" at Settings, Messages and make sure that "YOU CAN BE REACHED BY IMESSAGE AT..." has selected your USA mobile number (i.e. there is a tick next to it, if not, then tick the USA mobile number).
3. Reboot your iPhone (turn it completely OFF and back ON).
This should activate iMessage.