Pay back the airtime

I recently moved from contract to prepaid. Last Saturday I loaded R30 airtime and mtn gave me bonus airtime so my balance was about R60. And I tried to make calls all day but I was told I have insufficient funds. But my balance showed i had R60. I finally made a call for under 2 minutes that evening. The next morning I had a nil balance. And no, the airtime was not spent on data charges because I have wifi and my data is always switched off and I'm not subscribed to anything. When I tried to call customer care I was told I reached my limit to speak to a consultant and they gave me a date when I can call back to speak to a consultant (I was so shocked and so mad that I didn't even take note of what date) and if I really wanted to speak to someone I should call 083 173. I haven't called 173 since being back on prepaid! So how did I reach this limit? Nah there's a fine line between loyal and stupid. In an emergency I probably wont even be able to use my cellphone. So where did my airtime go? Why wasn't I able to speak to a consultant? The last I spoke to one I was on contract. I'm really disappointed. I should port or spend my life complaining.