RingCentral has four basic fax packages, which are less expensive when the customer is locked into a contract. The most popular package is the Fax 500, which is $7.99 ($9.99 w/o contract) for 500 faxes per month. Under this plan, there is one dedicated fax number for the office. The Fax 1000 and Fax 2500 include more faxes, plus the ability to receive faxes on a PC or smartphone. You can send faxes via email with the Fax2500 plan. The Fax ProUnlimited includes all features and a unique fax number for employee.
I believe the reason it is slightly less expensive on its face for the ProUnlimited plan is because larger companies will have more fax numbers. All plans have the same price for additional fax numbers, which is $4.99 per month. A vanity number, e.g. 1800-MyFax4U, will carry a $30 one-time fee.
The Fine Print: