Please stop calling and harassing me

These people are harassing me, telling me that I owe standard bank R30 000. Everyday I recieve a call from them and sometimes they don't talk when I answer they hang up. They say they are a law firm that represents standard bank. This has been happening for 6 now. I have been asking them to send me proof of the account on my email and Im always been told that their system does not send statements but they will get a manager to send me that statement. What makes it worse is that the guy that I spoke to last week told me that my account was handed over to them last month. Yesterday I received a call from a lady from standard bank offering me a credit card, I went on with the process to find out if I qualify and guess what, I do qualify. Please can you stop calling and harassing me