When login into your Office365 account you will get asked to validate your login. This is a mandatory security mesure that will ask you to validate each new login from a phone or an app. If the landline phone number is selected for this mandatory step, you will need to answer that landline EACH time you are asked to validate. If you are unable to answer the call on your landline, you will be unable to access your O365 accounts. This includes access to emails, office apps (words, excel, powerpoint), teams, and much more. To ensure you get access to everything right when you need it, make sure to use a mobile device you that is accessible at all times.