There should be an option to disable Office Phone as a verification option
On the MFA settings portal, there is no way to separate office phone from cell phone when choosing the verification options that will be presented to users. The only option is "Phone call". However, when users go into the verification options on their user profile, they can choose to do MFA via "Office phone" or "Mobile phone".
There needs to be a way to disable Office Phone as an option so that users cannot select it. It is not secure as a verification option.