Sideline does not offer the ability to "hide" or "restrict" your Sideline number from the recipient of the call (the person you're calling).
To keep your number anonymous, dial *67 using your personal device (not Sideline) followed by the 10-digit phone number.
Note: If you try to use Sideline to dial out with *67 then the phone number, you'll receive a pop-up error stating that your call could not be completed (seen below).
Questions? Submit a request.