18numbers
1890 = Local Call Rates always , normally voice numbers
1991 = Discount rate during the day and local off peak. Requires contract ...eg IOL Gold
1892 = Local Call Rates always , non contract internet numbers
1893 = Flat Rate ...these are the cheapest if you use the Internet on dial up during the day for any length .
The first 3 all cost the same per minute off peak . The difference is during the day .
Looking again at your bill, how much time do you spend on the internet off peak and peak (separately) in a month ?