Insist to give your ID number before giving any detail, harassing you daily

This consultants phone me on a daily basis telling me they phone with regards to an overdue hospital account. They insist that I have to give them my ID number over the phone. I refuse to give my ID number over the phone. If they phone me with regards to an overdue account, they should have my details. I asked that they should sent me a mail. But they keep on insisted on my ID number and phone me every day, Harassing me. These consultants are the rudest consultants. By means on the hospital account. The medical aid and the hospital made co-payment mistakes and make it the members/ patients reliability problem to get it paid/ solved.