If it is Teams for free, then it means you use Microsoft personal account. In this situation, you can refer to the step to change the phone number:
- Sign in to the account that has the phone number you want to change.
- Select Remove next to the phone number.
- Select Add phone number to include the number to the account.
If you are a user in your organization, you need to contact your admin to change the phone number which associated to your account. The admin can modify your information in User page in Microsoft 365 admin center.
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