I have a prepaid plan that has unlimited texting, 200mb data & unlimited incoming calls & unlimited calls after 6pm & on weekends but my calling seems to not work. It keeps saying my account balance is too low when i try after 6pm. I tried reseting my network settings on my iphone but that didn't help.
What could be happening is that you probably are on a city that is not local for your number so remember the free calling time is just for Local, so if you are out of your home city and you dont have balance it won't work.
If you're at home, probably you'll need to call *611 to make sure your number is local on your city.