Sedgewick Claims Management — unethical behaviour

I have a claim that I'm making payments toward for a car accident I was involved in a rental car from Avis in 2015. This company contact in February 2016 threatening to turn the account over to collection unless I pay in full or set up payment arrangements. I set up the payment arrangements monthly on the 28th of each month. Sedgewick won't accept payments online, won't keep my information on file, and won't answer the phone when I try to call to make the payments. Each month on the 28th I call the number they provided to make the payment and each month I get a different voicemail from a different person who never return my call. It's unethical to threaten to turn my account over to collection but when I try to make the payment to not all the only method I have for making the payments.