These adapters use Rockwell chipsets, when using the recommended init-string, mgetty will recognize DCD as high and force a hang-up.
Here are the lines added to /etc/mgetty+sendfax/mgetty.config:
port ttyG0_03
init-**** "" \d\d\d+++\d\d\dATQ0V1E0 OK AT&S0&D2&K3S0=0 OK