I don't have an Apple watch but I don't believe there is a RingCentral mobile application specifically written for the Apple Watch. Therefore, if she is having phone calls go to her cell phone number, rather than a RingCentral app, that would cause declined calls to go to her personal voicemail. Based on that scenario, there is no way to change that, other than having her answer/decline calls in a RingCentral application.
Just verified from the App Store regarding the RingCentral mobile app (Watch is not listed):
Requires iOS 9.0 or later. Compatible with iPhone, iPad, and iPod touch.