On modems with a single TTY for both control and data, we cannot use
the TTY to load connection status once we're in connected mode:

    Connection through a plain serial AT port (ttyUSB2)
    (ttyUSB2): --> 'ATD*99***2#<CR>'
    (ttyUSB2): <-- '<CR><LF>CONNECT 100000000<CR><LF>'
    (ttyUSB2): port now connected
    Connected bearer '/org/freedesktop/ModemManager1/Bearer/0'
    Modem /org/freedesktop/ModemManager1/Modem/0: state changed (connecting -> connected)
    Simple connect state (8/8): All done
    checking if connected failed: Couldn't check current list of active PDP contexts: No AT port available to run command
    checking if connected failed: Couldn't check current list of active PDP contexts: No AT port available to run command
    checking if connected failed: Couldn't check current list of active PDP contexts: No AT port available to run command
    ...