Does att shut off phone immediately if I don't pay on due date?

Prepaid service is shut off when your 30 day plan expires. Prepaid does not have a bill, with prepaid you refill your account. If your due date is the 13th, that’s probably the last day of your plan meaning service will cut off at midnight of the 13th if you haven’t refilled your account sufficiently to renew your plan. If you don’t have enough to renew your plan, you will have the option of purchasing bridge pay which extends or plan for 7 days or you can wait until you get paid. You just won’t have service until you either renew your plan or purchase bridge pay.