During peak traffic, you want your customers to talk with someone, not get a busy signal. Use the direct-trunk-overflow feature to ensure callers terminate on a DID/ANI ring-to number.
To add direct-trunk-overflow to a routing plan:
Control Center displays a list of all services assigned to the accounts under your enterprise ID. If you have more than ten services, you can view the rest of your services by clicking one of the blue linked Show numbers at the top of the list or you can click the blue down arrow at the bottom of the list to add ten services at a time to the view.