Airtime disappearing off my phone

Good day<br> <br> In January I decided not to upgrade and instead switch to pre-paid, I then lost my R1000 airtime but I was warned about this in advance- so it's okay. <br> <br> I went away this weekend and bought data and airtime, since I knew I would be away from wifi. Instead of using my data, my airtime was used. On Friday night, I bought R60 airtime and in the morning my balance was R0.00 and I never even used my phone throughout the night. Yesterday, I bought R30 airtime just to test it and my balance was R0.00 after having made no calls/sending SMS's since I was at work the whole day. I did have data bundles. <br> <br> This morning I bought R10 airtime, having about 180MB of data... went onto Instagram and checked again and my airtime was on R8 and my data balance unchanged. <br> <br> Over the past few days I have lost about R200 in airtime and I would really like to know where it has been going? At this rate I could have just upgraded and carried on with my contract where I had no issues. <br> <br> I am using an iPhone 4S and I have checked a few reviews and my Cellular Data APN has been set to internet instead of mymtn. <br> <br> Could you please help me? <br> <br> Thank you kindly,<br> <br> Candice<br> <br>