Dial-up vintage BBS access with agetty and a modem

I'm trying to setup a 80's service with Mystic BBS (that runs on telnet), including a dial-up access with a U.S. Robotics modem connected to the serial port of the server. I succeeded with mgetty: /sbin/mgetty -D -a -x6 /dev/ttyUSB0 This brings the Ubuntu login on the remote client, that I can replace with the output of telnet on localhost, configuring the process that is started upon the connection is established. **The problem is that mgetty requires the caller to enter an existing username on the server** before he can get to the welcome screen of Mystic. So, I switched to agetty: it has an auto-login feature that will skip the first step and it will directly bring the user to the telnet session using a conventional non-admin linux user. Then the user will login or register to the BBS service. Unfortunately, I can't get it to work: agetty -I 'ATE0Q1&D2&C1S0=1\015' ttyUSB0 vt100 With this command I can connect from the client, but nothing is shown on the terminal. The connection is stable, but nothing seems to be transmitted. If I send some strokes from the terminal and I monitor /dev/ttyUSB0 with *screen* on the server, I can see garbage characters that arrives, but everything is scrambled. Same thing if I send strokes from the *screen* session on the server, some "brick" characters are shown on the client terminal.