help with collection problem please!!

I co signed for a apartment and have been being harassed by debt collectors being that the person I signed for did not pay his rent. It has been 2 years and when it first started I tried to set up a payment plan but being a student and struggling myself I could not afford more than $100 a month. I called today to settle but I guess they ran my credit and said I have a credit card with available credit that I can pay it in full with. I do not want to pay one debt to create another. My balance is only $2200 and I told them I could send them a check today for half. They would not accept this and stated that they are going to take legal actions. Can they do this when I am trying to pay it? Also I have NEVER received anything by mail, they told me they sent it to the original lease holder and when I told them I have very little contact if any with that person they lady told me that sucks to be me. Can they do this? I want to fix this but do not know how I can not afford that amount at once what can I do?