Billing begins when your first line is activated or seven days after your order is shipped, whichever comes first. You should expect to receive your bill within a few days, after which it will be due. Your payment method will be charged on the due date. In that first bill, you will pay for your current month of service (the days since you signed up through the remaining 30 days).
The day that your bill is due will be the same each month. If you incur additional usage charges (such as International calling or roaming), the charges will appear on your next month’s bill. The same applies to any late charges incurred.