MTN-SA blocked my contract sim cards without notifying me and I do not owe them

MTN-SA blocked my contract numbers for calling without alerting me and I still do not know why.

I paid the monthly required amount, however 4 days after they have blocked me from calling. This is bad service and I need my contract numbers to be unblocked.

I do not even need any explanation from them because I paid as required. All I need is the numbers to be unblocked.