VMS Help Sys Parameters, DISABLE_UPCALLS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    DISABLE_UPCALLS is primarily a debugging aid. It allows the
    system manager to disable threads upcalls of specific types
    for the entire system. The value is a bit mask, with the bits
    corresponding to the upcall types. The upcall types are defined
    in the definition macro $TMCDEF.

    This special parameter is used by Compaq and is subject to
    change. Do not change this parameter unless Compaq recommends
    that you do so. DISABLE_UPCALLS is a DYNAMIC parameter.