Hi,
Try the following to see if either resolves the problem:
[https://www.t-mobile.com/support/devices/android/oneplus-8/wipe-cache-partition-oneplus-8|Wipe the cache partition] and check if it works OK on ''every'' call. As it is random allow a bit of time to be sure
Try using the phone in [https://www.t-mobile.com/support/devices/android/oneplus-8t-plus-5g/safe-mode-oneplus-8t-plus-5g|safe mode] and check if you can hear OK on ''every'' call.
If you can then a ''downloaded'' app may be the cause of the problem. The trick is to find out which one. A ''downloaded'' app is one that you ''downloaded'' and installed on the phone and was not pre-installed on the phone
You will need to uninstall each ''downloaded'' app, one by one and test the phone in normal mode between each uninstall. When it works OK then the last uninstalled ''downloaded'' app was the cause. Being random this may take a while.