Fuzzy sound at other end when phone is on speakerphone

Hello all,
I have a Fairphone 4, with the latest update to Android 15, and the security update from 5th January. Lately, whenever I use my phone with the phone app, and put it on speakerphone, the sound at the other end becomes fuzzy and distorted. It seems to get worse as the call continues. This seems to be a software problem related to the phone app, as when I use the voice calling option on WhatsApp and use speakerphone, there is no problem the other end. I have tried turning of VoLTE calling, and Wi-Fi calling, but this makes no difference. Is this a problem with the latest update, or is there some problem with my phone? Thanks in advance for any help or advice.