To find out information about your billing, products/subscriptions, and anything related, head to your Account Menu > Billings & Logs > Products.
Here you will find detailed information regarding billing including whether you are being charged monthly or annually as well as the next billing date for each product/subscription that you purchased.
From here you can also upgrade from smrtPhone Standard to Pro or from smrtDialer Single Line to Multi Line.
If you change your subscription, several changes will happen. You can learn more about what happens to billing when you change your subscription level here >>