Pathetic MTN service of calls not being allowed!

I have noticed that other MTN clients have the same problem. I top-up via STD Often I try to make a call and then get a voice message saying I don't have enough airtime to make the call. If I check the balance I fo have sufficient. This is unacceptable and obviously an issue. Horrible service. I've rebooted my device with the same result. I have also raised this issue with the MTN call centre and they are not bothered to have a supevisor get back to me with a solution.