I'm pleased if a case change has solved the issue for this user. However, we have anecdotal evidence in 2 situations where the problem has persisted - egregiously - one with an iPhone 11 and an Apple silicone case (the kind where the ther is no crossbar on the bottom near the speakers), and one with no case at all (but it does have a screen protector.
The most successful strategies I've seen to date are to a) quit all other apps on the phone prior to making or taking a call (this has worked to our amazement in several instances) and b) keep the base of the phone away from the palm, parts of the hand, or any flat surface or base or charging station. Annoyingly, holding the phone upside down and speaking into it as if it were a handheld mike seems to work best.
We have not tested this with FaceTime - the upside-down technique would likely yield questionable results.
PS We have -not- seen this problem on any iPhone 11 Pros.
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