Im not sure that assumption is valid. You included far too many variables that are unproven (number of calls, people taking calls) as indicated by your frequent use of the word "assume".
A more reasonable extrapolation would be to compare the number reported per 1000 in the Aussie population as a whole and extrapolate that per 1000 of jw in Australia. Those numbers should be fairly easy to find. I will hit google and see how it compares out