How to reconfigure invoice numbering?

Question

Currently there is a different numbering scheme for each billing office and we would need to go to a simple incremental invoice number for all billing offices. However, none can have the same invoice number. Using parts b and c all the same, will the system know not to duplicate invoice numbers or will they be duplicated across billing offices?

Resolution
To have unique invoice numbers, part A of the invoice number needs to be different for each billing office.
It is not possible to have a single incrementing invoice number across all billing office without any differentiation by billing office.