Step 1: Go to the TMS URL: 

Step 2: Choose the correct account

Step 3: Click on the PBX Manager menu item.

Step 4: Click on the PBX Settings sub-menu item.

Step 5: In the very first dropdown box, select the number you would like as your Default External Caller ID.

Step 6: Uncheck the box that says Allow Personal Caller ID.

Step 7: Now click the Apply button. This change will take effect immediately.