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

    Support: L1,L2
    Default: L1,L2             Value: Address string

    The address that is to be inserted to the beginning of the address
    extracted from the NSAP address. This is valid only if mapping is
    not manual and the parent circuit entity's access type is Modem
    DA. It is used in conjunction with modem access suffix to form the
    complete dial sequence.