iPhone doesn’t produce sound, but only when calling

Since recently, my iPhone has stopped making any sound when in a call. It makes sound completely normally in any other situation - I can listen to voicemails too. When I’m in the call menu, the ‘audio’ button is greyed out. The problem appears both when I’m using (off-brand) bluetooth earbuds, as well as when I’m not using any headphones at all.

Is this a problem anyone’s encountered before?