Hi teresamigacz,
I understand how important it is to be able to communicate well with others, and how audio quality can play a big role in that process.
I'd suggest performing a test:
• Navigate to: Settings > Phone > Wi-Fi Calling
• Disable this feature
• Test a few consecutive calls to see if this behavior persists.
If the behavior does not persist, this behavior is related to the Wi-Fi calling feature from your carrier, and you'll want to contact them for support.
If the behavior does persist, try these steps:
• Go to Settings > Cellular > Cellular Data Options > Voice & Data
• Choose: LTE, VoLTE Off
• Test a few consecutive calls to see if this behavior persists.
If the behavior does not persist, this behavior is related to the VoLTE calling feature from your carrier, and you'll want to contact them for support.
If the behavior still persists, you'll want to contact Apple Support for further assistance: Get Support
Enjoy your day.