If you own an iPhone 12 (Pro) and sometimes cannot hear the other party, you might be in luck: Apple has extended a repair program for the iPhone 12.
We had already made you aware of it here. For a certain percentage, the speaker in the earpiece seems to be defective and will be repaired free of charge when the defect occurs. Only models of the iPhone 12 or 12 Pro are affected. The mini and Max models are not covered.
If you believe you are affected by the defect, check Apple’s service page for further instructions. Other ongoing repair programs can be found here in the overview.