This specifies the AFI, IDI, and preDSP to use when constructing an
 NSAP from a Phase IV address.  This is used when a Phase IV address is
 specified in an address tower.

   PHASEIV_PREFIX addr-prefix

 For example:

     PHASEIV_PREFIX 39:840:800AB738

 If not specified, the Phase IV prefix defined in the export file is used.
 If the export file does not contain a Phase IV prefix definition, then
 the Phase IV prefix set using the SET DEFAULT command is used.

 See the PHASE_IV_PREFIXES topic for details about Phase IV prefix values.
 See the EXPORT_FILE topic for a description of the Phase IV prefix control
 line.