Threatening phone calls!!!

<p>I had a contract with Cell C,The contract was handled by Shapiro Shaik Defries and Associates and it has been paid up.I made the last payment on April 2017,I even got a statement from Shapiro Shaik confirming my account being paid up.All of a sudden i recieve threating calls from Nudebt management claiming that i still owe Cell C,I tried to explain to them that i have paid up my account through Shapiro Shaik Defries and Associates as it was handed over them.I emailed Nudebt management a statement from Shapiro Shaik to show that i have paid my account in full,they didn't even bother to respond my email yet they keep on calling me,threatening to blacklist me for the account that i don't owe.When i call them to ask if they recieved my email,the guy called Sandile answered a phone and he was unprofessional and clueless.He said to me the statement is not a prove because it shows transactions and it might be of something else not Cell C but my statement clearly i was paying Cell C.I honestly don't trust them.</p> <p>Please help how can i solve this issue?</p>