Mixer settings for voice recording: It is required that DAI Mode (alsa control number 50) is "DAI 1" - only recording, or "DAI 2" - recording and playback simultaneously. "DAI 0" is only for playback. There's also "DAI 3", but I don't know what this is.
Depending on the recording source, Mic1 for headset or Mic2 for handset have to be set accordingly via the Mic Selection Mux, which is alsa control number 53. Also, set Capture Left Mux and Capture Right Mux (65, 64) to PGA.
Below is an alsa state file, which can be used to put the device into voice recording mode. It's advisable to have a programm connected to e.g. /dev/dsp, else the device might crash. Change settings via:
alsactl -f voice-recording.state restore