Calls can only be heard on speaker mode

Hello,

Review all of the steps in this link:

If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker

Good luck.

If you’re receiving calls and they’re constantly being heard on speaker and would like to disable you can Go to settings/Accessibility/call audio routing and make sure the setting is set to automatic. You could also go to your current call and hit the speaker icon and choose your preferred call audio setting but would have to do so after you answer every call you receive. You can also make sure you haven’t accidentally enabled the mute option while on a call or FaceTime.

it might be a troubleshooting issue where the comment above has the most insightful link that would help you resolve said issue.