Phone numbers are billed monthly on the day of the month you purchased the number. If that date does not occur in the next month, the billing date will be changed to the last day of that month.
For example, if you purchased a number on October 31, you would next be billed on November 30 because there is no 31st day in November. You will continue to be billed on the 30th going forward (Dec 30, Jan 30) until February when the billing date will switch to Feb 28th (or Feb 29th in a leap year).
On the date you purchase the number, you are charged for a full month, no matter when you release the number. So if you release the phone number a week after purchasing it, you would have already been billed for a full month.
For full pricing details, see our pricing page.