QoS is the set of techniques to manage network resources, You can troubleshoot by following below steps to check whether you have any issues at your end.
Check the internet connectivity and make sure you are using a congestion-free network if you are using Plivo endpoints to make/receive calls.
Check whether your device’s microphone & headset are working fine.
Check if there is enough bandwidth for the network.
Check if the bandwidth available would be enough for the number of simultaneous calls happening from the network.
Is the Voice packets prioritised over the data packets. If not, for better call quality it is suggested to prioritise Voice packets over the data.
Are the endpoint users using any other applications or websites that are consuming the available bandwidth.
You can contact Plivo support team so that we can help you troubleshoot and fix the issue.