A microphone could work together with a radar; correlating the two signals may help tell the sound from the noise.

I suppose that at certain radio frequencies human bodies are mostly translucent, while the small metal membrane is still resolvable. When a head of a person covers well the acoustic signal (the phone is at the opposite ear), a microphone is helpless, but a radar us still fairly usable.

Note that in this study the radar targeted the whole phone, not the membrane.