Hi Dan,
This can be acomplished by creating mapping between the two entities. Now, you can't add mapping between this system relationship (Phone Call 1 : N Campaign Responses), so one possible solution would be to create a new 1:N Relationship of your own and set
the mapping between the fields.
Mappings will only work between fields of the exact same type (Single Line of Text -> Single Line of Text, Lookup - > Lookup, etc).
Let us know if that helps