Intermittent Bad Call Quality

Need help troubleshooting intermittent call quality issues. System is using Freepbx 2.11 with Asterisk 1.8.11.

Users experience call quality breakup. For some of the users I have seen Jitter delay but not for all users. There is no evidence of packet loss. The problem can occur on internal calls or external calls. The PBX is hosted remotely but the bandwidth connection is 10M/10M and I haven’t seen any issue with bandwidth whilst monitoring the connection.

I suspect that the cultprit might be a switch on their local network, but I don’t know how to pin down what is causing the call quality problems. Any suggestions on how to find the cause?