Unsatisfactory PSTN call quality could be related to slow/unstable Internet connection or poor PSTN termination.
First you must make sure VN to VN or echo test, 600 type calls are of good quality.
If they are not, this is not a PSTN termination issue, please review our other "Voice Quality Issues" articles.
If within network calling provides satifactory audio quality, the issue is indeed poor temination related. In this case you must do the following:
a) Call the number you are having problems with.
b) Call another number with a different area code/prefix (other city or country).
c) Determine if you had the same problem when calling both numbers or just one of them.
d) Forward your test results to our support team for further review. Do make sure to include the call history of
the faulty calls.