MTN Customer for 15 Years - application for new contract rejected with no explanation

I have app**** for a new contract with MTN, my existing contracts are all due for an upgrade. I have been an MTN customer for 15 years (originally with Nashua mobile). The contract is being refused on the grounds of a negative credit score. I have checked my credit score with TransUnion and Clear Score and my credit score is clear... MTN will not divulge what the problem is or how it can be rectified. I need a new service provider...