Assuming that the outbound path is affected (on a poor quality call, the customer can’t hear the agent well), I recommend that you first capture traffic on your TX PBX, which you can then analyze when you get a report of a bad call:
Look at the RTP from the agent for lost or greatly delayed packets. If you see such, you can try to determine whether the problem is local (weak or overcrowded Wi-Fi, or link saturation from unrelated traffic), the internet connection (e.g. heavy traffic from users on the same node), or routing over the internet. From the agent’s PC, you could run three continuous pings, one to the local router, one to ISP first gateway, and one to your PBX, each recording to a file. When trouble is seen, stop the pings and examine the relevant section for loss and delays.
If ping to ISP gateway is good but ping to PBX has heavy loss or jitter, then an intermediate PBX close to the agents will probably help. (First make sure that the connection between the two PBXes is clean.)