Blockquote …Vodafone is by far the worst in voice quality.
Have you contacted Vodafone? If there is a difference in sound quality between the two providers very likely it is not the phone (or the software) , but something with the SIM card(s).
Anyhow, this solution (and the update) are not for sound quality, but for “no sound at all” and “dropping of phone calls”.
As described in the information about the update:
So, my recommendation is to first test with another SIM card. Try one of family of friends to see what the quality it. Best would be to avoid the carriers you are using (Vodafone and the other). If with different SIM card the issue is solved contact your carriersto request new SIM cards.
If the problem remains with different SIM cards contact Fairphone Customer Support again and tell them you did what I mentioned in my previous reply and replaced SIM cards but the quality is still bad.