The microphone is the problem here. There are two microphones on the iPhone XR: One is built into the display assembly, and the other is part of the Lightning (charging connector) port assembly.[br]
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Generally, if a microphone is going to go out it’ll be the one on the bottom near the Lightning port, which is good because it’s a less expensive part to replace, even if it’s more of a pain to do.[br]
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Occasionally though, as weird as it is, it’s not a defective part so much as Airdrop receiving being turned on causes the microphones to malfunction, so I would try turning that off and restarting the phone first.