Charged for roaming while i did not activate the service

I went to Rome and Oman in the current year, while I was there, kept my phone on only to access wi fi and therefore use whatsapp to communicate with people back home, since I didn't activate roaming. I got calls and I thought maybe its because of the airtime I had since I am on top up contract and I can make or receive phone calls with this additional airtime I bought. To my surprise I get home to a bill I didn't sign up for, airtime has not been used and now theres an extra amount on my bill. Called MTN customer service and she first of all assumed that when I said I went abroad it was in the SADC region, and she kept saying if I went there then that why my phone kept receiving phone calls although I didn't specifically choose the roaming option. She also mentioned that I don't specifically have to activate roaming, mtn automatically does that without my consent and this and this must have been explained to me when I got the contract which was donkey years ago. She then suggested that she put through a complained on my behalf and someone will call me in the next 45 days. I waited and it has not be a while and no one has come back to me. The lady I spoke to is Nadia Masondo and the Ref no for the call is ********** 10. I am disappointed and hence the reason for my on renewal of my contract. My work requires quite a bit of travelling and I cant subject myself to this