This didn't seem right since the previous month the bill showed a bunch of calls but with zero costs. This month many of the calls during the month were zero cost until the last week when each call was billed.
I spent a while with the call centre on this problem who were not able to resolve it and said they would refer it to the technical team. They were supposed to call me back but two days later I have heard nothing.
I wouldn't normally post issues like that here but it seems I am not getting resolution from the proper channels. Either something is wrong with their billing system or I am not understanding how my wife's plan works. Certainly even towards the end of the month she would enter the USSD code for balances and it should ample free minutes left which would make her feel comfortable with continuing to call. I guess not!