Need to update my phone number, software won't accept old number even when I enter it.

Hello

Is this for your personal account? If you go to https://account.live.com, can you sign in? If so, go to Security > Manage how I sign in to update your security information here.

Also note, for personal Microsoft accounts, Microsoft now says you can no longer add a phone number as a new security method. This method is being phased out. See official document here. You will have to use the other methods, such as email, authenticator or passkey.

https://support.microsoft.com/accounts-billing/manage/microsoft-to-stop-sending-sms-codes-for-personal-accounts

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If you cannot access it because you cannot receive the code, look for an option like “Show more verification methods” to see if you have a backup (e.g., a phone number or alternate email).

If you don’t have access to any of the options listed, select “I don’t have any of these.” This will begin the recovery process. As part of that process, Microsoft will let you add new contact information — but only after a 30-day waiting period.

This 30-day delay is intentional. It’s a security safeguard designed to protect users in case someone else tries to hijack their account by changing recovery info. The wait gives the rightful owner time to notice and stop unauthorized changes.

Unfortunately, this policy cannot be bypassed. Microsoft explains it further here.

https://support.microsoft.com/account-billing/-your-security-info-change-is-still-pending-or-you-can-t-access-this-site-right-now-microsoft-account-message-cbd0f64f-02d9-45d2-90c3-2375e5a72e52#:~:text=While%20we%20understand%20this%2030,phone%20number%20or%20email%20address.