Library /sys$common/syshlp/nclhelp.hlb set, modem_connect, line, rate_select  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: High              Value: High or Low

    Which of the line rates is to be used if none is specified when
    a call is setup. high indicates that the value of the speed
    characteristic be used, and low indicates that the value of the
    alternate speed characteristic be used. This characteristic is
    supported only if the characteristic modem control is full, and if
    the characteristic modem options includes rateselect.