Dealing with your lender or servicer: Student loan

I pay my student loan servicer once a week, each and every week. I owe roughly double the amount I took out in student loans due to interest. But I don't miss payments. They started calling me to catch up on payments of additional interest. They started to call me and I asked them on three occasions not to call me because I use my phone for business and they were calling me in excess of 4 times per day. They said they would stop calling and email or mail me as per my request. However it has not stopped, I receive phone calls from them constantly. It is making me anxious and it disrupts my business use. Please make them stop this is awful.