Why pay for an installation fee when I already had a fibre line paid directly to Vumatel???

I signed up with Vox in June last year on one of their promotions. of free installations but when i signed up I had told them that i have a line with Vumatel already that I have paid for fully all i need is for their fibre services to be connected. now I tried cancelling end of March because I was moving houses. they are now harresing me wanting me to pay for a cancellation fee because i had free installtion. which isnt true. I have an paid invoice from Vumatel showing that I paid for my line Prior to getting fibre from Vox

so Now i must pay twice..

Can someone assist me because everytime i try to explain this i get disregarded and get told I have to pay the cancellation fee