Unhappy customer !

Usually when my data is finished Mtn sends me an sms saying so.. This is now the second time that mtn hasnt sent me an sms saying my data is finished or expired.. I had 17 gig left from the 30 gig i got when I upgraded and I still checked it yesterday.. No sms saying it expired and suddenly R500 rands worth of airtime is used because I had no idea the data was finished.. This is the second time this happened costing me a lot of airtime.. When I only buy one gig they sms me twise to say my data is almost finished or expired but the moment i have a lot of data they dont let me know when it expires or at least send me an sms a day before it expires.. Very unhappy with mtn at the moment as I decided not to switch to vodacom after the last incident cost me R1000 worth of airtime and mtn did nothing about it, deciding i'll give them another chance and this happens !!