Why do I have poor voice quality when calling the PSTN?

The first step is to install PingPlotter and let your VSP check your ISP connection. If the trace route looks satisfactory and meets all VoIP standards then then the issue is indeed poor PSTN termination.

Call and test the number you are having problems with.
Call another number with a different area code/prefix (other city or country).
Determine if you had the same problem when calling both numbers or just one of them.
Ensure that you include the call history of the faulty calls when sending the support request to your VSP.