How to chase away a loyal clients

I have been with MTN for many years and been a MTN Prestige client. Recently I got a call from MTN to upgrade my cell contract. So I took the advantage of upgrading my contract and my Daughters. I have always been on contracts that have unlimited calls and data so I requested the same. Obviously for my daughter I took something much smaller. Upgrades went through etc but whilst I was on a call I got a notification that my minutes where up and my call stopped. Eventually I got hold of MTN and found out they used an outsourced company to do the upgrade and that they cannot correct the upgrade or assist me. I am now unable to make calls, have an incorrectly loaded contract that is 36 months instead of 24 months and I have to fight with someone MTN forced on me to resolve the problem. Honestly shocking I mean if they don't want to assist clients why do they bother actually asking us to renew. My contract is with MTN why can they not sort this out?