VMS Help SET, TERMINAL, Qualifiers, /MODEM  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /MODEM
       /NOMODEM

    Specifies whether the terminal is connected to a modem or
    a cable that supplies standard EIA modem control signals.
    If your terminal has the MODEM characteristic, entering SET
    TERMINAL/NOMODEM automatically logs you out. The /MODEM qualifier
    and the /COMMSYNC qualifier are mutually exclusive.