Microphone stops working regularly

iPhone 11 Pro Max (dual sim) on iOS 15.4

I have repeated issues with the phone's microphone somehow getting disabled by itself in apps like WhatsApp, Threema, LINE or FaceTime, when using both phone lines of my phone.

The microphone is clean, it works by itself (tested via recording audio & video) but very frequently whenever calling or receiving a call via web / wifi on the apps listed above, on either of my two lines, the microphone is not working all of a sudden. This happens on every WiFi network I may use, not just one. All app permissions are granted, microphone is enabled for all the apps in question. Also, when using "regular" cellular service for calling, the microphone works normally, on both lines.

The only way to "solve" this is to reset the phone's network and privacy preferences every time, then setup everything again, then the microphone works in every app, only for the same issue to start again one or two days later. Really annoying!

I realize there are many third-parties involved (router, internet provider, phone, apps etc etc) so I'm kind of expecting another round of "it's not us it's them" on the IT (non-)problem-solving merry-go-round......

Anyway, just sharing and hoping maybe someone out there was having similar issues or experiences and was able to identify and resolve the issue. Thanks in advance.