Each company has a number of different plans, which will usually consist of: a) standard monthly fee, b) number of free minutes, c) per-minute prices for calls after the free minutes (within the network, to other operators, to land-lines, etc.) d) text messages, etc., etc. Add in bonus programs that lower the rates across the board, pre-paid packages for minutes, text messages, etc., and special rates for numbers you frequently call, and you could probably write a college thesis on the pricing plans for all three companies, computing different algorithms for a theoretical monthly bill. Of course, after a month, it all becomes obsolete.