If you see this window when you try to activate Office online you need to perform a phone activation. Please click on “back” and continue to the next step.
Now choose the phone activation option and click “Next”.
Now choose your country and call the provided number and follow the steps to point where you enter the installation ID and receive the confirmation ID which you put into boxes A-H.
If you see a message about Phone Activation being no longer supported disregard it, this message is displayed to everyone who is not a subscriber of Office 365. Phone number can be found here on official MS website.
After typing in the Confirmation ID from MS hit next, product key will be verified and that’s it. Office is successfully activated and ready to use.