you should try to disable 4.75
then you might see that it is now the next codec (5.9) which has lots of bad frames.
if that’s the case, it means that either your C/I thresholds are wrong.
or it measn that there are some severly interefered zones in which even the most robust codec can’t fight.
keep in mind that the most robust codec is the one used in the WORST areas of your network, so you would expect to see the worst radio quality. Therefore, conduct a drive test in the worst cells, find out where the worst C/I are localised, and do an voice quality (in UL and in DL) to ensure the codec is unable to keep up with the interference. Try modifying the codecs to see the impact, etc.
If the experiment shows that 5.9 is showing less bad frames than 4.75, then it might mean there is an issue with 4.75. But it can also mean something else 🙂