To protect your data, a phone number can only be added to a single Microsoft account.
So, after signing into a Microsoft app and you see a message asking if you want to change number or continue, it means this number is already associated with a different account.
You'll see this if:
You created a Microsoft account with that number in the past, but you forgot about it
One of your family members is already using this phone number with their own account
This is a new phone number for you. Since mobile carriers can reuse phone numbers after 30-days, it's possible the previous owner still has that number associated with their Microsoft account.
Since the number was only recently added to the other account, your only option is to use a different mobile number or continue setting up the app without adding a mobile number.
To fix this you have the option to use a different mobile number (the Change option), or you can continue to the next step and choose not add your phone number at this time.
Use the following guidance to help you select the best option.
Use the Change option if:
You want to add a different phone number to be used with this app so you can access all the features of the app.
Use the Continue option if:
You only have one mobile number, and since the Microsoft account you’re using with this app is also yours, you want to use it here as well.
You’re setting this app up for a child or another individual who doesn’t have a mobile phone, but you want them to have this app and you want to use your phone number.
If you select Change number, you’ll be brought back to the phone number entry screen where you can select a different mobile number to use with this app.
Select Change number.
Enter a new mobile telephone number.
Follow the remaining prompts to add the number to the account you're using with this app.
If you select Continue, the mobile number you added will remain on the other account and you’ll be allowed to sign in to the app without adding a mobile number. We’ll ask you at a later time if you want to update this choice in case you change your mind.
Note: If you select this option, some of the features on this app might not work (such as easily finding friends and colleagues using the Teams app).
Enter the password for this account and follow any remaining prompts in the app.