I've only been a RingCentral customer for a few weeks, but during that time I have been testing the website, softphones, and iPhone apps extensively.
I upgraded my iPhone 5S to iOS 8 as soon as possible last week and I don't see any problem with the RingCentral app. The app claims to be version 6.5.0.
I have not tested conference calls or faxing because neither of those are important to us at the moment.
The only problem I've noticed is that if I answer an incoming call on my computer (softphone) my iPhone keeps ringing for a long time after I answer the call. I doubt that's related to iOS - it appears to be an internal timing issue at RingCentral.
Let me know if there's anything in particular you would like tested.