Vumatel hounding me for payment - Afrihost needs to pay

While I thought Afrihost was one of the best Service Providers, they quickly revealed their true colors. Vumatel installed my line for me in March 2018. Earlier this year 2019, vumatel called me to make payment for the instalment, but according to my Afrihost Tax Invoice as well as what was discussed this was covered by their "promotion" they were running. My Invoice and Credit Note clearly states it's discounted and yet Afrihost insists their promotion was over and I must settle the account with Vumatel. Why give me a legal Tax Invoice stating the discount provided and 1 year later dispute it? After several emails back and forth with numerous employees at Afrihost, I am still hearing from Vumatel to settle my account, why am I being hounded for doing nothing wrong and trusting the information Afrihost provided? Why does Vumatel not take this up with Afrihost?