1. Sippy has a page in the main menu to let you manage DID numbers:

2. Click "Add" button to add a new DID number

3. Set your DID number in the fields "DID" and "Incoming DID/CLD" then click the "Save & Close" button.:

Sippy Softswitch uses Incoming DID/CLD for authentication of an incoming call by CLD when you have specified one.  

As an additional measure Sippy recommends that you use the Vendor & Connection fields in the "Authentication Rules" section.  This is for authentication of incoming traffic only from a specific Remote IP (IP from which DID provider's traffic will arrive at Sippy).

2. As an additional security measure Sippy recommends that you authenticate incoming traffic by remote IP also.  To achieve this, you need to open the "Vendors" menu and add a new Vendor & Connection with the IP of the DID provider from which traffic will arrive at Sippy:

3. Then, open the "DID Pool" page -> properties of some DID and set an appropriate "Vendor/Connection" in the "Authentication Rules":

4. Then click on "Assigned to" and assign your DID number to some Account or IVR application; this will result in the incoming call to the DID number being routed to a specific Account or IVR application:

5. If your call should be routed from the "DID" to the registered Account, check to be sure it is set in the "Incoming Routing" assigned to the DID account: