Disturbing threatening calls

I had a contract with MTN which I cancelled due to bad service . I spoke to one of the call center agent around April to cancel my contract which was not done however I was told it was , I then received a call from their lawyers advising me I owe them which was a total surprise , I called their call center again on the 07/05/2017 and I was transferred to their retentions spoke to some rude lady who confirmed the contract was cancelled and I don't owe them anything , but I'm still getting calls from their lawyers advising me I still owe them money , and this calls are everyday , they call me on my office number , cell phone number like all the time . I regret the day I took out this contract as MTN doesn't care for their customers .