Library /sys$common/syshlp/nclhelp.hlb set, x25_access, reachable_address, address_extensions  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: True          Value: True or false

    Specifies whether the "called address extension" and "calling
    address extension" fields are to be included in the outgoing call
    packet.

    Example:

    set x25 access reachable address x121 address extensions true