Phone lined blocked monthly yet not in arrears

I am at my wits end with MTN!!! Every month for the past 3-4 months they have been blocking the lines linked to the contracts i have with them. This being done while the contracts are fully up to date. Going to the MTN store does not help , they just say they don't know why this is happening as its always paid on time. Yet month after month its the same damn issue, if MTN can not stick to their end of the bargain they might as well stop the contracts with no further fees due by me as they are not sticking to their end of the contract. I am tired of having to deal with this monthly and also each they block the line it messes with my credit record and most likely decreases my credit score. I WANT THIS FIXED ONCE AND FOR ALL ...STOP MESSING WITH MY DAMN CREDIT RECORD MTN. SORT YOUR ISSUES OUT IMMEDIATELY