Hi Sravanthi, Immadi [EMR/SYSS/PSS/HYDR]
Thanks for reaching out to Microsoft Q&A.
As you have lost your mobile, you can go to security page- https://mysignins.microsoft.com/security-info and delete your old mobile information
Once you have deleted old mobile info, please click on add sign and add details again by clicking on add sign in
Once you click on add sign in , you will be able to see steps to add authentication
For more details: Reference: https://learn.microsoft.com/en-us/entra/identity/authentication/howto-mfa-userdevicesettings#manage-user-authentication-options
Please do not forget to "Accept the answer” and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.