Back it up to the PC card before doing anything, in case you don't like the results.
Under Operators, Positions, set them as DLC's and "Save All". After the system restarts, any lines in pool 70 will be automatically assigned to the Operator phones, which you may want to remove.
Then renumber your main operator to whatever the telco sends for the main line, and probably add coverage for the main operator to the other ops.