How do I stop personal mobile number being visible to all in Basic Info?

How do I stop personal mobile number being visible to all in Basic Info?

When viewing employees' basic contact info,  the work phone and mobile phone numbers are displayed.  In reality, the work phone is the landline and the mobile phone isn't the work mobile but the number recorded under personal mobile.  I don't want employees to have access to seeing each others' personal mobile numbers - how can I change this to display work mobile instead?