Measure quality, not bars
Check retries, channel use, noise, latency and loss during a real call.
Reduce radio contention
Channel planning and sensible widths often help more than adding power or another access point.
Walk the route users take
Test the call while moving between work areas. A stationary speed test will not expose a poor handover.
Official technical sources
These sources support the technical explanation. The right fix still depends on the environment where the problem appears.