Airtime usage while data is available

I have recently taken a contract phone at an mtn store. I collected the phone on Wednesday, the 9th of May. As I was being assisted, I enquired about the airtime that I bought the day before, and it was confirmed that I will not lose it. The lady also confirmed that the free 30GB has been loaded, and I saw it. I kept checking balances, and I was relieved to find my airtime. The anytime minutes, sms and data, package of the contract, were also loaded, apportioned accordingly. Thursday late in the afternoon, I realized that my airtime is depleted, zero balance. I phoned mtn, and I was informed that my airtime was used up on internet, and I asked, how is it possible, as I had data and I was told that the 30GB was only loaded Thursday morning. I then went to the store on Friday, and explained my situation to the lady that assisted me before. She confirmed that my 30GB data was aready loaded as I left the store on Wednesday, but she couldn't understand why my airtime was used. After waiting for a long time, she changed the tune, and said that my airtime was used on internet, and she does not know why it was used, and not the data. I asked her is it safe for me to buy airtime going forward, and she shook her head, that she doesn't know. I was so disappointed when I left the store, here is someone who I thought will be able to assist me, but it turned out that I was wrong. It was a very painful experience, on the first days of taking out a contract phone! The only consolation is the new phone that I'm using.