SUSPENSION OF OUTGOING CALLS

I had received an sms from MTN with the amount owing on the 3rd September 2015. I then (On the same day) went into my bank account and paid MTN for the services. Today I had received another sms from them, stating that they are suspending my outgoing calls as they did not receive the amount. I made sure and double checked that the money did go off the account.. My payments with MTN are debit order and it is not the first time that they have done this. I want to know why the person responsible for going through their bank account cant do his/her job properly and why they have to be so ***********. I am not happy with MTN anymore and really am thinking about going to Cell C to get them to pay my contract out.