Hello, thank you for reaching out to Microsoft Q&A.
The Account Recovery Form requires adequate information to prove that you are the owner of the account. The Account Recovery Process can be a time consuming process, but providing as much info as possible from the start of the process will increase the possibility of being verified by the automated process. You'll need to keep trying until you're verified by Microsoft.
If you don't have the correct information, you may not be able to get through account verification using the Recovery Form.
I think recovering your old account in this situation would be very difficult since you no longer remember the correct details for it.
You might see the 'Suspicious Activity' message if someone tried to access your Microsoft account from a different region or location. For security reasons, Microsoft temporarily blocks your account and asks them/you to verify your identity using your alternate contact methods, such as phone number or email address. Since you only have the old phone number to verify your account and it is no longer in use, it will be very difficult for either of you to recover/access the account. In this situation, your account is completely lost. So, there's no need to worry about the account, as they may not be able to access it or you.