To update an extension's caller ID, navigate to Connectivity > Inbound Routes to determine what number you want to use.
Next, navigate to Applications > Extensions, and edit the extension (Pencil and paper button under Actions).
In the General tab, you will see the field Outbound CID. Add or replace the number in that field.
Hit "Submit" at the bottom right of the page
Then hit the "Apply Config" in the top right.
Conduct a test to confirm the change was made.
As long as your office doesn't force a global caller ID (not a default setting,) each extension has the ability to call out with their own caller ID.
If you want all extensions in your office to call out with the same caller ID you would need to contact the support team to make this change.