Bad customer service

I had a wireless fibre line with VOX, which worked very well, until some trees and other issues caused problems and eventually had no signal. this was fine as fibre was being installed in my area. I called Vox and asked to upgrade to a fibre line, as soon as it was available. about a month later, I get my fibre, but i get billed for my wirless and my fibre line. i contacted them to sort this out, eventually the wirless gets taken off my account. My question is, how do i get the refund for the months i was charged double? i have been sending mails since the 9th July, and the only responce is, we will escalate it. Calling Vox is usless on these queries, i just get told to send a e-mail. Vox fibre services are pretty good, but their customer service is the worst.