Bad phone sound quality during phone call

Hi community,
I have just purchased a FairPhone4 for my girlfriend. The problem is that the sound quality coming from her is very bad. Not only when calling me, but anybody else.
What is strange is that, using a Bluetooth device, I can hear her very well.
Bad sound quality = bits and pieces of what she says are missing and each time when I talk I can hear all ‘S’, ‘Sh’ and ‘Ch’ sounds as an echo 1 or 2 seconds later.
I upgraded it to the latest software.
I contacted the helpdesk and performed the microphone and speaker tests and they were OK.
I replaced the SIM card (because helpdesk advised me to do that) but the problem did not dissapear.

Am I the only person with this issue?

Kind regards, Vincent

2 posts were merged into an existing topic: Call quality and noise cancellation (regular calls, handsfree, Signal voice messages and video calls)