Sound quality drops sharply when telephoning. What to do?

I have a Huawei P8 lite 2017 and unfortunately can't afford a new smartphone at the moment. But that's not the problem but the only sound quality when I'm on the phone or on discord or in another voice chat is rapidly dropping. So normally everything is normal and the sound sounds good. But as soon as I go into voice chat in a game or make a phone call, the game sounds like a toaster. What can I do?

If your toaster has a better sound, join it

I've already tried. Now I need a new toaster