Microphone crossed out during audio or video WhatsApp calls

I have the latest iOS (14.8) and latest WhatsApp version on my iPhone 6s.

WhatsApp has clearly access to the microphone (I can for example record and send an audio message).

However when initiating or receiving audio or video WhatsApp calls, the microphone icon on the WhatsApp call is grayed (crossed) out.

This happens also after restarting the iPhone, with WhatsApp being the only started (live) app.

I tried to reinstall completely WhatsApp but that didn’t help either.

I contacted the WhatsApp support but they couldn’t provide any useful hint.

Thanks.