o  already in loopback

    The modem connect line entity specified in the command is already
    in loopback mode. Check that you specified the correct entity.
    Reissue the command with the correct name.

 o  line not connected

    You have specified the value remote for the mode argument, yet the
    line is not connected. Connect the line (using the dial command)
    and the reissue the startloop command.

 o  line disabled

    The line entity must be enabled before loopback.

 o  constraint violation

    A startloop mode constraint has been violated.