Disappearing Airtime

Good day, I would like to state that MTN South Africa is *****ing from me. I am using a prepaid SIM that I purchased as a prepaid service from day one. Two days ago I recharged my account with a R30 voucher. I made a call and was billed R2.28 for it. About 2 hours later I tried to make a call to a non-MTN number and the MTN automated service informed me that I did not have sufficient airtime to make this call. When I checked my airtime balance my R27.72 airtime that I had paid for had now been replaced with MTN's free airtime that expires at 23H59 of the same day. That free airtime is USELESS because it can only be used to call MTN numbers ONLY hence the reason for forking out hard earned cash to have the freedom of calling whomsoever is connected irrespective of chosen service provider. Secondly, on several occassions where I have had a good value of airtime (R10.00 plus) I would try to make a call and be informed by the automated voice that I \do not have sufficient airtime to make a call". I called the MTN customer service to sign out of the free airtime promotion ( mind you"