In order to configure a DID from a custom carrier, perform the following steps:
1. Go to Settings> Numbers.
2. Click "+ Existing Number".
3. Enter the number into the "Number" field.
4. Fill in the "Endpoint" field (if there isn't one you may leave this blank)
5. From the "Carrier" drop-down list, select your custom carrier
6. From the "Assigned to" drop-down list, select the entity to which you would like to assign this number (for example, assign to a User)
7. From the "Users" drop-down list, select the User to whom the number will be assigned.
8. Click "Save"