Turned my celldata & data roaming off stillcharged

i am trying to write something to CellC on the customer service section of their webpage ,wow I can only write about 20 words???? I moved my contract to Cell C via Vitality mobile after having a contract with MTN for 20 years and after a few years I am so frustrated. The international roaming rates are 3 to 4 times more expensive??? So when I travel I turn off my cellular data and my data roaming, I have also turned off my Wi-fi assist on my IPhone 6s . So this is a clear instruction to CellC that I don''t want to be billed for international calls nor do I want to receive them . I connect to the Wi-Fi of the hotel and received calls and made about 3 calls ,but when I was out of range of the wi-fi this was not possible. I have just Found out that these call were charged to my data ????? I called CellC and they say that if the sim card is in the phone they will always use cellular data??? This is theft ,I clearly didn't want to use data or pay these ridiculous roaming rates . This is ******* and I expect CellC to credit me for this!!!