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