Create a value list - showing all extensions dialled from each number. So create a self join relationship(number dialled), and the value list will use values in field "extension", but including only related values from the self join.
Then use Value Count to count the number of values in the value list.
So, a calculation field (number) will have
ValueCount (ValueListItems ("Database Name";"Value List" ))
This should result in the number of extensions that have dialled each number...