EP-T21 microphone volume low during calls

Hello!

I have an EP-T21. When I’m in a phone (or whatsapp) call, the other party only hears me extremely low and they have to put their device volume to the maximum (this for ALL other parties, not just one - so it’s not on their side, but rather on my device side).

Any idea how to correct this? Unlike Windows, there is no microphone volume that I can find on my Samsung phone.

Thanks for your help,
Gilles.