Finally we test the audio by phone call. The following is the command for reference.
root@swi-mdm9x15:/mnt# microcom -E /dev/ttyAT
//Call out
atd0928XXXXXX;
OK
CONNECT
+CMTI: “SM”,5
ath
OK
//Call in
RING
+CLIP: “0928XXXXXX”,129,0
ata
OK
ctrl + x to leave