Microphone and camera failing seemingly at random every few minutes.

Hey y’all,

I am completely stuck with random issue a friend of mine has with her iPhone 7: Seemingly at random, the microphone and other call-related parts of her phone stop working. For a few minutes, everything works and then, suddenly, everything stops working. The issues appearing are the following:

• When calling normally (no FaceTime, WhatsApp etc…), she can’t hear the other person nor can the person hear her. It doesn’t matter, whether there are headphones plugged in or if the speaker is turned on. Nothing works.
• On FaceTime, the same thing happens on both audio and video calls. Additionally, I can’t see her but she can see me. However, when she opens the camera app, both cameras work.
• Sometimes, she can’t record voice memos on WhatsApp. When this is the case, WhatsApp gives her an error message saying “WhatsApp can’t record your voice memo […] if you keep getting this error, restart your phone.”
• Sometimes, she can’t listen to WhatsApp memos either. When she clicks on play, the crossed speaker icon appears as if her volume is turned down to zero.
• Restarting the phone sometimes fixes the issue for a few minutes.
• Settings -> Privacy -> Microphone is all set correctly (all apps have access to the microphone apart from Google Maps)
• Settings -> Accessibility -> Audio/Visual -> Noise Cancelling is turned off
• Settings -> Accessibility -> Noise Reduction is also tunred off
• Settings -> Mobile Data -> WIFI-Calls is turned off
• She updated from iOS 13 to iOS 14 last night. The same problems appeared on both iOS versions.
• I connected to her phone via TeamViewer Quicksupport when everything was working. I walked her through a few essential settings when all of a sudden, everything stopped working again. At that moment, the video from TV QS stopped streaming and our call continued but I just heared my own last two words over and over again.
• Turning bluetooth or WIFI on or off in the settings app doesn’t make a difference.

Has anyone ever had similar issues? I have no idea what is going on. For me, it seems to be some kind of random software problem since it seems unlikely, that all of these parts just stop working at once. Also, since WhatsApp and Facetime calls also don’t work, I don’t see a problem with the connection/carrier. TL;DR: HELP! :D