As such rules seem to be being applied after they agreed to help out, just explain why it's thought necessary, and maybe phrase it in such a way as 'we are all in this together and could they please help by going outside for personal calls, and to try to avoid/dissuade members from following them out'. Perhaps they could say something like, "excuse me, but I'll be back shortly", to members before they go out. If they object, well they don't have to volunteer; and new volunteers can be made aware of the guideline when they join up.