Call audio transmission through the SIM7100 PCM interface is massively noisey and distorted

Playing audio files to the modem's ALSA device results in massive distortion when listening at the other end. In addition to distortion, there is extreme noise when there should be silence. This is true for music or speech, in particular replaying speech recorded through the same PCM interface in the other direction.

Adjusting AT+CTXMICGAIN, AT+CTXVOLEX, AT+CNSN, AT+CNSLIM or AT+CECH does not alter the degree of distortion or noise.

Noise appears when playing silence.