RXQual and BER are related. The RXQual is just a certain scale, and each RxQual correponds to a certain amount of BER.
FER is when a speech frame is erased by the TRX, because it couldn’t be decoded properly.
Usually, at low BER, the FER is good, but if BER degrades, then FER suddenly becomes bad.
It’s because the TRX could re-build radio blocks into a proper frame even when there is a bit of BER. This is thanks to convolutional coding.
But that works up to a certain BER.