Sony WF-SP900 in combination with Fairphone sounds bad

I have a FF4 and the Sony WF-SP900 headphones sound ok with music from the phone or the internal storage, but when the phone is connected they sound as flat as if they would be compressed to 32kbps. Why is that and how can I improve the sound-quality when phoning?

While phoning, they use a different protocol, HFP or HSP for example, because they need to transfer the microphone signal too, that reduces bandwidth for the audio.

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