15 meters is pretty close to the edge of a typical cell edge in an office environment where MR33 would propagate it signal in a donut shape. That may be your issue, particularly for VoIP. One possible issues you can address: - APs at the ceiling level can see each other pretty hot and reduce TX power to adjust what it thinks is high-density deployment; however, client devices at the floor levels are too far to use lower TX power levels; try adjusting the minimum TX power in the Radio profiles to be no less than 14dBm (the typical max supported by smartphones) If it doesn't resolve or improve your issue, I'd revisit proper coverage via an RF validation
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