I have received official notification from Fairphone that this is now a recognised problem and they are working on it.
(Fairphone)
Apr 8, 2021, 11:40 GMT+2
*Dear *******
Thank you for contacting us, I apologize for the inconvenience this issue is causing you and for any delay in our reply.
It seems your complaint concerning the poor sensitivity of the microphone is due to a software issue. Our Product Team has already been alerted and we are urgently working towards a resolution.
We would appreciate it if you could please allow us a reasonable amount of time to investigate this. We do want to offer some advice and so if possible, as a temporary workaround, we can suggest using a wired or Bluetooth headset when making a call (as your daughter is already doing). We realize this is not ideal so please know your issue has the highest priority to be resolved.