Debt collector won't work with me

I've been out of work for 4 years now due to me being disabled. My only source of income is SSI & Social Security Disability. And from what I understand, SSI and Social Security Disability are exempt from creditors, debt collectors, garnishment, levy, attachment, etc.

So then, I explained my situation to the debt collecting rep that I'm currently drawing disability, and that I've been out of work for four years as a result of me being disabled. But this particular collection agency still insists I work out a payment plan. A payment plan I simply cannot afford right now due to me being disabled. If all possible, any advice on my matter would be greatly appreciated. Thank you.