Worst Service Provider for a CONTRACT

My fiance took a phone contract out with MTN last year Jun, in September he stopped getting his Airtime, sms's and Data but we've been paying every month. Then MTN start calling saying his account is behind. We weren't able to see if they deducting because MTN blocked the Sim Card so he couldn't do anything with the Sim in the phone. In December we went to MTN in Zevenwacht Mall the guy that helped us said he can't see anything wrong he can see we've been paying the account and then all of a sudden everything was working on the phone only for 2 weeks then everything went down again. In this year debt collectors call us that the MTN account have been given over to them because we failed to make the payments.

I'm so furious cause MTN is a *******.

Can anyone just advise me what I can do regarding this matter