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

       /PPPD_CALLOUT=image-name

    Specifies the name of the protected shareable image activated
    by PPPD. This image contains the routine PPPD$OPEN_CONNECT,
    which notifies the network service that a new physical transport
    exists that supports PPP. If the image resides in the SYS$SHARE
    directory, enter the file name of the image without the suffix
    (such as, PPPD_CALLOUT instead of PPPD_CALLOUT.EXE); otherwise,
    enter the system logical that identifies the location of the
    image.