you are charged your access fee/contract fee in advance, calls and call credits in arrears. it happens on every mobile service and has done forever. you will also find if you look at your home phone bill, you are charged calls in arrear, but your line rental in advance

it's exactly the same as what happens with house rental - you pay in advance.

you pay your access fee to use the network before hand, and your calls afterwards (and any call credits are applied after you've made them
It doesn't happen with every network, I'm with 3 on a monthly cap and my phone charges are debited the following month after they were incurred.

It is very frustrating if they do not advise you that it is going to happen, although it is probably written in some terms and conditions in fine print somewhere if you look hard enough.