I'm using MPE 1.7.3 in the USA with an AT&T GoPhone SIM, it's a prepaid SIM that after every text message or call sends a information text with your current balance.
I use MPE 1.7.3 in the UK on Orane with no problems whatsoever.
In the USA however, MPE is unpredictable, it becomes unresponsive, I have to keep on unplugging the USB cable and reconnecting to get it all to work.
The only thing that is different is the AT&T SIM. Are things different in the USA?