I have the same problem as well. I have an iPhone 11 and this only happen when I’m using speakerphone. When I’m using headphone or having the phone right on my ear, there is no issue. Speakerphone voice call or FaceTime calls are a problem. I have spoken to Apple Support and they made an appointment for me to go to an authorized repair center. I went on Friday but it was no help. They couldn’t find any problem.
For the first time yesterday, I experienced it on the receiving end. I was on my iPad and my daughter was on her iPhone 11. She kept cutting in and out because she’s using the speakerphone (she normally uses the headphone). It is rather frustrating to not hear what she was saying every third word.
If anyone has a solution, I’m open to anything. Thanks.