Greetings Erichlub,
Looks like you've done some great troubleshooting already to help with isolating the issue. Great job! There are a few more steps you can try to further help with resolving this issue with the microphone on your iPhone. Check them out here: If the microphones on your iPhone aren't working - Apple Support. Some you have done, but here is a list of the ones not mentioned that you may want to try:
- Remove any screen protectors, films, or cases from your iPhone.
- Clear any debris from the openings of your iPhone.
If the issue remains after this step, you'll want to reach out to Apple Support directly, as you've gone through the other recommended steps on that page. Be sure to let them know what resource was used and the steps you completed. You can reach out to Apple Support directly by clicking here: Contact - Official Apple Support or over the phone by calling the number that is most local to your region: Contact Apple for support and service - Apple Support.
We hope this information helps you out.
Best wishes!