MTN bonus airtime

MTN South Africa has a promo or whatever they call it where they say you get airtime bonus matching amount which you have recharged with which is supposed to expire after 24hrs, but that's not the case. After I recharged with R50 today which covered my airtime loan the bonus airtime disappeared after I made 3 calls which were 31sec, 25sec and 1min21sec and mind you it's a second time it happens. You send an email query to their customer service they turn things as if you crazy. MTN is taking advantage of the customers and their network coverage has became nonsense this days. They not delivering