Vodacom wont allow me to upgrade to an iPhone

Been with Vodacom for almost 30 years. My profile says I am able to upgrade my number however, it does not allow me to upgrade to an Apple iphone even though two years ago I upgraded to Apple iphone. They are saying my credit score is low and it has been low since 2014 but I never had an issue with upgrading to another phone. I have never defaulted on a payment. This is what you get for being a loyal customer.