Starting June 2017 You will be charged for all those "virtual users" you made if they don't have a digital line/number assigned to them. So if you have 10 users and are only charged for 5. Your bill will double! I found this email notice by accident in my spam folder.
Over the years, RingCentral has steadily enhanced your service with significant capabilities to enable multiple forms of communication including voice, video and audio conferencing, team messaging and collaboration. These capabilities allow your employees to communicate anywhere, anytime on any device.
All employees connected to the RingCentral solution have access to these services. RingCentral has not been enforcing licenses for employees that are using the system on mobile devices without a digital line (Mobile Users).
As of your June 2017 statement, Mobile Users using the system without a digital line will be considered part of the total licensed users and will be charged the same as users who have a digital line. For your account, this is $44.99 per user. Please note that you have the ability to add digital lines for these users at no additional charge (excluding taxes and fees).
You can find more details here, including how to identify Mobile Users without a digital line on your account. Last month you had 6 on your account.