working with this company for over 10 years

I have been working with this company for over 10 years to repay my student loans. This company continually calls me after I have repeatedly sent the letters requesting an adjustment to my payment amount as well as requesting that they do not call and to conduct my personal business with them in writing only. This company has representatives that are rude and use threatening language, as well as do not post my payments to my student loan account.