Threatening to get my name blacklisted

<p>Hi Peter, i have a cell phone contract with Ismart from 2015 they delivered a phone and a sim card contract one ,the phone did have even six months so it started not working but the sim card was , They collect it on March last year and I had to wait for it for months after I longed lot of complains because they no longer sending airtime n the sim card.After they send me a secondhand phone telling me that other was missing in the store room and the guy promise to send me all my airtime that they were not sending with the new sim card so that one I used to have will no longer work.Im having their phone wth no network simcard again and again they send me one not working but they expect me to pay the full amount but everytime when I call they promise to sort this even now they dont they call want meto pay so Ive told the that am not going to pay until they sort this out now they threatening to blacklist my name .Please help me</p>