Mtn cellphone contract

To say I'm hurt will be an understatement. Last year in November 2023, I've decided to take a cellphone contract with the mtn in order to build my credit score. I have never missed a payment. But to my surprise I was told that im a bad payer. I've provided my bank statements to their store to prove my innocence but I still didn't get help. I've been calling their customer service but im still not getting help. I'm afraid that this is going to affect my credit score.