nearestObjects is what you're searching for. The resulting list is ordered by the distance of each object of given type to given object, means, closest object is the first element in the list, aso.
as trigger condition, and the activation would be something like
count nearestObjects [position phone1,["Man"],2.5] > 0
hint format ["%1 has discovered a satellite phone", name (nearestObjects [position phone1,["Man"],2.5] select 0)] ; phone1 addAction ["Tap satellite phone", "tap_phone.sqf"]