problems with collection agency

i had an exxon mobile card that ended up going into collections...i set up arrangements with the collection agency and at the same time the account was still open and i was receiving bills from exxon mobile. i made payments to the collection agency and my payments were reflected on my statement from the credit card. In september i called to postpone one of my payments with the collection agency and they told me there was nothing they cold do to postpone the payments(i called about 9 days in advance). so they attempted to take the payment out of my account and the funds were not available. i started to receive nasty messages from the collection agency and I called them to arrange a payment. they told me that they were going to have me arrested for writing a bad check and then when i said i never wrote a check and that i tried to have my payemnt postponed the person hung up on me. i called back and i spoke to someone who told me that if i paid 100.00 a month that would be fine. so i set up the payment and i never received anything from them that stated that i authorized them to take money out of my account and when i just got my bill from exxon mobile there was no payment reflected in this statement. my question is what rights do i have? can i call mobile and tell them i don't want to deal with the collection agency anymore since they are rude and threatening and there's no proof anywhere of this last payment. Everytime i call the collection agency they are rude, nasty and end up hanging up on me. what should i do?