Airtime disappears

MTN is a joke, you load airtime and make one call only to make another call and be told you have insufficient airtime to make a call....after just one call. Check balance and it sure enough its a R0.00 after loading R30 and making one short call and you have enough data so it can be that. i have to now switch off my phone and back on so that my airtime will return. This is a new trick i have learnt which means MTN have been *****ing my airtime for the longest time. Its just such a pain that i have to turn my phone off everytime i need to make a call, this is ridiculous because there what happens if there is an emergency and i need to make an urgent call??? This is not on guys and i have been with MTN for over 10years!!! Please sort yourself out or i will be another loyal customer that will be walking out!!!!!!