Hello, 9448. Thank you for sharing your question on the Apple Support Communities. We understand the situation you are noticing and how you are unable to hear clearly when making calls with your iPhone. We know that staying connected is important and we can certainly help you with this.
To get started, we would first suggest that you check each troubleshooting step listed in the following Apple Support article:
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support
If you are still noticing the same issue after this, we can help you more. To confirm, does this only seem to be an issue with audio sent to the earpiece? If you place a call on speaker, or use a connected headset, do you notice any similar issues?
It may also be a good idea to check for carrier settings updates and iOS updates. These last two articles can help you with this:
Manually update your carrier settings on your iPhone or iPad - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
We hope that these troubleshooting ideas help you get back to making phone calls normally again. Please message us back here if you need more help with this.
Cheers!