Absolute pathetic service from MTN

I've been a customer for MTN for years, I recently renewed my contract and told at that time to not have a limit on my account, it's a business phone and I've had experiences in the past where I'm trying to do something and can't cause off no "airtime". I tried sending an MMS today and was unable to cause of no airtime issue again, which i was told was sorted out when I renewed my contract. Called call centre who was not helpful at all, told that a zero limit may not resolve the airtime issue, I'm sure they have their reasons but I cant understand why it is so difficult to do stuff on my phone, and then charge me for it when my bill goes off my account? I still can't send an MMS and now have to make time to go to an MTN store.