BGC IT,
Bandwidth shaping seems like a good idea, but in fact it makes troubleshooting more difficult, because it is artificially manipulating the airtime that clients get. How much of a client's throughput is being limited by bandwidth contracts, a true congestion issue or bandwidth shaping is impossible to tell. Properly designed VOIP clients will use WMM to automatically prioritize the traffic over the WLAN, so they do not need this mechanism. Bandwidth shaping was designed primarily for multi-tenant organizations that need to guarantee specific percentages of bandwidth to their tenants, but if the RF side of your WLAN is not optimized, it could end up reducing bandwidth by constraining traffic to what is left of it. Merely enabling this does not magically fix anything.
I would first attempt to try to fix the RF for all users and then try to narrow down issues to specific clients, instead.