Why can't Employees see each others' phone numbers?
For privacy reasons, the default setting for the Employee's phone and email information is set to only allow Managers to see them.
Each Employee can sign into their account and go to their "Info" section to change their phone and email options so all other Employees can see it if they choose. Then if shared, their phone and email will appear in the "Staff" section for other Employees to see.
You could send a message out to your Employees encouraging them to change their settings so other Employees can see their information. But this is an option each Employee must decide to change based on their own privacy concerns.