Harassment

I have a payment arrangement in place and yet they will not stop harassing me. I received a letter that I owe a balance from Dec 2019 yet I've been making payment as arranged from 20 February 2020. I will not be harassed this account was handed over because I lost my job and I'm still unemployed if someone isn't doing their job that's not my problem the monthly payment I keep up to date I've never missed a payment