I can confirmed this issue is occuring on four new iPhone 11, one new iPhone 12 mini, and two new iPhone 2020 SE.
Cut-out behavior is consistent when loud speaker is enabled during a call, the microphone on the same device cuts in and out, causing the conversation to break up on the receiving side. When switched back to ear speaker, the conversation is clear on the receiving side.
Another consistent behavior is when the case on these phones is removed, the problem "seems" to go away, but that hasn't been tested for an extended period of time.
There are 3 microphones on these iPhones, (1) hole next to ear speaker, (2) hole in left bottom speaker grill, (3) hole near front camera. I wonder if a different microphone is used for recording with loud speaker on versus with ear speaker during a call.
Having a case does not block any of these microphones. Something with the iOS software is causing these cut-out issues, and evidently this problem is wide spread across many iPhone models.
Apple engineers: please replicate this issue in your lab ASAP and provide a patch fix. Problem has been around for at least over a year.