Log into your 3CX webclient
Navigate to the 3CX Admin Console located in the lower left corner of the webclient.
From the Admin section select "Voice & chat"
Inside the Voice & chat section click on your Voxtelesys SIP Trunk or select "Edit Trunk" from the three dot icon.
Inside your Voxtelesys SIP Trunk navigate to the "DID Numbers" tab, to add more numbers/DIDs press the "+ Add" button.
Enter the DID number and click off the number box or select "+ Add" to add another number.
Be sure to "Save" when you are done adding and assigning your new DID numbers!
If you want to verify, order, or port numbers you can do so from the Voxtelesys Portal: