To begin with, the INIT CHAT string should not be BLANK after enabled the ttyM1. It is
"" \d\d\d+++\d\d\dATZ OK
Then, the mgetty output should be like below even though there is no line connection:
[root@ACS6048-0520086903 ~]# tail -f /var/log/mgetty.ttyM1
02/13 14:41:54 yM1 waiting...
02/13 15:41:54 yM1 checking if modem is still alive
02/13 15:41:54 yM1 mdm_send: 'AT' -> OK
02/13 15:41:55 yM1 waiting...
02/13 16:41:55 yM1 checking if modem is still alive
02/13 16:41:55 yM1 mdm_send: 'AT' -> OK
02/13 16:41:55 yM1 waiting...
02/13 17:41:55 yM1 checking if modem is still alive
02/13 17:41:55 yM1 mdm_send: 'AT' -> OK
02/13 17:41:56 yM1 waiting...