Help with DDL

I have two DDL

DDL_A

Items:DepartmentA, DepartmentB

DDL_B

Items: 1233215000, 5672006000 (I have lots of numbers that starts with 123...Total 10 digits and lots of numbers that start with 567... and so on)

How I can force the user if they select a number that starts with 123.. from DDL_B the display item for DDL_A always will be:DepartmentA

Thanks