BEING BLACK LISTED FOR A DEVICE I DONT HAVE

I'm so hurt and disappointed by MTN, I've been a loyal customer but the treatment I'm getting is heartbreaking, MTN sent a bill of R6000 threatening to black list me for a device that i don't have, I've been waiting for them to contact me as promised but they didn't they just send bills. i bought a phone online, received it but it froze then i took it back i was told to wait for 3 weeks, till today i didn't receive their call.