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I spoke with the RingCentral Mobile product manager and was informed that we do not provide any official RingCentral APK file outside of the scope of the Google Play Store and Apple Store downloads since we only support devices which are interoperable with those app stores. If your device is not supported by one of these app stores, you should contact the store provider and your device manufacturer to support your needs. (RECOMMENDED)
If you want to take the risk, I found lots of results online with a search, but that would be 110% up to you as well as you assuming any and all risks (NOT RECOMMENDED).