At a user's detail sheet, there is an option to block access to emails and phone numbers in the mobile app. If checked, this user will be able to access customer contact information.
If unchecked, the customer's contact information and/or contacts will disappear in the mobile app.
Tip
You could turn this on for (for instance) the team leaders and not the other technicians. That way, only one person on site can contact the actual customer with questions or problems.