Open the "SIP Trunks" tab.
Add a new SIP trunk.
Under the General tab, add your Main Trunk No as +1 (your DID) ex. +17019299797
If you have more DIDs, click on the "DIDs" tab at the top and add other DIDs in the same format.
Click "OK" at the top to save changes.
Any Outbound Caller IDs you set will need to have the +1.