Will Porting a Phone Number Confuse the Billing System?

I'm a fairly new GoPhone customer -- my second month is about to start. The problem is that I got several text messages saying that my plan can't renew because my account balance is too low. But there is more than enough money in the account (I checked the balance both by logging on to payonline.com and doing ^777# on the cell phone). The only thing I can think of is that, since I ported a number from my previous carrier, the system is looking at the original number that came with the AT&T SIM card -- not my ported number. When I log on I use the ported number.