Paying for call charges on a top up contract

<p>In April 2016, we went to Makro Meadowdale to upgrade my wife's contract with Vodacom. She upgraded from a open line contract to a Top Up 350. When she received her monthly bill from June 2016, she was charged for around R50.00 worth of Vodacom to Vodacom phone calls. How can this be as she is on a top up contract? Are there any fine prints that we are unaware of as we always thought a top up contract works on an airtime basis whereby once your airtime is finished, you have to go buy more airtime, load it and then only can you make calls again?</p>