James, the bandwidth you require will depend upon many factors, including call dispersion, call holding times, grade-of-service offered to the callers etc. Also affecting the choice will be how you currently receive those calls (1800 number, normal number) and the type of telephone equipment you have. What changes are you willing to make?
I’m in Aust too, Adelaide. if you want to discuss it further fax me a contact (phone or email or fax)
My fax is 08 8110 5100