Managing personal Outlook.com account settings, security, and privacy
Welcome Maelstorm,
Thank you for explaining the situation. Since you were able to remove the old number but no longer see an option to add a new one, what you're seeing may be expected for your personal Microsoft account.
Microsoft notes that the option to add a new phone number may not be available for some accounts. Please sign in to your Microsoft account's “Security” page, select “Manage how I sign in”, and then select “Add a new way to sign in or verify.”
If “Phone number” is not offered there, use one of the verification methods that is displayed, such as a recovery email address, Microsoft Authenticator, or a passkey.
Microsoft's instructions are available here: Microsoft account security info & verification codes
Looking forward to your update.