Ask HN: Need advice on a great phone system

I've tried Google Voice, cell phones, Grasshopper, Avaya, RingCentral and several others. I have yet to find a system that I love. Each one solves some of the problems but the rest feels like a kludge.

Key for me are: - Simplicity to transfer calls - Compatible with physical (conf rooms and desks) and mobile phones - Simple to administer - Scales well for an office of 20+

Any advice?