Hello garywi,
Apple Support Communities is a great place for help with calling and audio issues with your iPhone. To confirm, are the calls ending unexpectedly without you or the other individual ending the call, or is the issue only with audio quality? If the calls are dropping without you or the other individual ending the call. Try the steps outlined in the following article.
If you see No Service or Searching on your iPhone or iPad
While the title of the article outlines issues with service, these steps may still be helpful. The next step would be to isolate the issue to being microphone or speaker related. The articles below can help with isolating the issue.
If the microphones on your iPhone aren't working
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
If the issue isn’t related to the speaker or microphone, and only occurs on calls. The If you see No Service or Searching on your iPhone or iPad article linked above can still be helpful.
If you continue to have issues after those steps you’ll want to reach out to Apple. You can find their contact details via the Get Support website.
Take care!