Credit Score too low to upgrade

I have been with Vodacom for over 24 months on a topup 175 or 165 contract and now that I want to upgrade, there is a problem with my credit score, requested how this credit score is calculated since I have not dishonoured my payments and with clearscore my credit score is 637 putting me in a bracket of having a good credit score.

Have been to the store, consulted telephonically and everybody at Vodacom has their own version of what could be a problem, sending me from: Upgrade department, Accounts department, Customer Acquisition etc.

May Vodacom assist me with a simple question, why is my credit score low in your department, if its low why do you continue keeping me in the same contract if I can't upgrade in the same Contract and why was I sent to customer acquisition if no one resolved the problem. I was told i could upgrade the 1st of January 2020 the earliest.

I want to know why can't I upgrade to a phone that's R300 or R380 a month but i'm currently paying R310 on my contract. And if my credit score were that low why did vodacom give me an contract in the first place?

I did try online but no one phoned me or anything. I want to cancel my contract if vodacom refuses to help me with an upgrade.