Open the official website setup.office.com
Sign in to your personal Microsoft account
Enter the 25-digit product key
Select the language and number of products in "Other Installation Options" according to your personal needs.
Select Phone Activation.
Enter the numbers from your activation window on the phone's keypad when prompted by the phone's voice.
The phone voice will give you verification IDs to complete the activation. Enter this in step 3.
When the phone voice asks "How many copies do you have installed?", type "1" on the phone's keypad.
IF THE PHONE ACTIVATION FAILED: In this case you will be transferred to a Microsoft agent. Microsoft is unable to provide support for product keys not sold directly by them. Contact us directly and we will help you.