The monthly billing plan charges your payment method on the anniversary date you signed up, each month. For example, if you signed up on May 15, the 15th will become your anniversary date each month, and your next invoice will be on June 15.
The annual billing plan charges once a year on the date when the plan was started and usually offers a discounted rate compared to the monthly plan. For example, if you signed up on May 15, the date 'May 15' will become your anniversary date each year, and your next invoice for plan charges will be on May 15 of the following year.
** Please note that any extras (i.e. additional numbers, add-on features not otherwise included, etc.) may be charged monthly as they were not included in the original annual payment.