RingCentral needs to allow non-RingCentral phones to configure for Message Waiting Indications.
There are posts dating back over a year and clearly the number of posts indicate that the users want this feature. I have an Aastra 6739i and it is fully configurable to have the MWI feature work.
If the programmers do not have it, the Admin Guide for the phone is here:
Just search on "Message Waiting" or "MWI". This is a pretty basic feature and we should be limited to the small selection of RingCentral phones.