This idea has been merged into another idea. To comment or vote on this idea, please visit ENTCOM-I-67 User Settings: Disable call waiting while on a call..
The 'Busy' or 'Do not disturb' options only really work when you have a scheduled call. Often, if I receive an unexpected call, I do not use these options. If another call then comes in while I am on the first call, it rings loudly in my ears (I use earbuds through my laptop) and can cause a loss of concentration as I manually divert the second call. Surely RingCentral should be able to determine you are already on a call and divert it to voicemail automatically.