No response or assistance because of strike

I cancelled my 3G contract in April because it expired. I then received a R12 000 account of which the data was extremely high. I struggled a lot with getting through to MTN call centre because of the strike and they just disconnect the line because that is the easiest thing they can do. I reached the call centre on 16 June and the person said he will load a dispute on the account. I then phoned again,but the call centre is closed. I sent another email on 22 June and again on 1 July but I still have had no response. I cut the sim card and threw it away because I cancelled the contract but I found out that they migrated it to a prepaid. Instead of doing their job properly they demand more money and go on strike. I dont think they deserve anything. I will not pay this because there is something wrong. In the 20 years I have had this contract I never had such a huge amount. Why now?