Direct Indial numbers are regular ten digit PSTN phone numbers. These numbers are the numbers that your customers use to call you from external locations. These numbers are commonly referred to as DID numbers.
Each SmartPABX is allocated with a group of Direct Indial numbers. How many you have depends on the SmartPABX package that you have purchased.
Each Direct Indial number can be configured to perform different tasks. For example if you have five Direct Indial numbers, it is possible to configure some of those numbers to be used as direct numbers for staff, some can be configured to be used for call conferencing rooms, and others can be used for IVRs, fax to email, etc.
Direct Indial numbers support multiple concurrent inbound calls regardless of whether the number is already in use. The only concurrent call limit for DID numbers is defined by the line limit associated with your SmartPABX package.