A simple alternative to sandbox testing - and which applies more widely that just to RingCentral - is to use a fax testing service like Faxbeep. It freely provides test fax numbers in the US and additional countries that instantly display your fax on a web interface. This makes it easy to verify your fax API implementation and debug issues. Faxes are generally retained for 30 days, but deleted immediately upon request for privacy reasons.

(Disclaimer: I run the service)