A cloud-based identity and access management service for securing user authentication and resource access
Microsoft has declared that SMS text or voice will no longer be supported for newly created Personal accounts and is phasing out SMS as a method of authentication and account recovery for personal Microsoft accounts since at least May. which a simple search will find many articles discussing online.
Microsoft to stop sending SMS codes for personal accounts | Microsoft Support
You're likely receiving this email since your particular account(s) are about to have the SMS ability removed, so Microsoft is trying to get you to update your security verification methods to the most secure passkey option or the only others that still exist in order to be able to recover or modify the security settings of your account once SMS is gone.
There's already many who've posted here having had SMS stop working for whatever reasons, which may be technical in their particular region, and who have found no way to recover their accounts, since Microsoft Support can no longer make manual changes to them and thus if there are no alternate methods of verification available, the account is effectively lost forever.
Ignore the warnings at your own peril, it's really that simple.
Rob