Allied Interstate practices

I live in Missouri and am very concerned about the practices of this company. I have a bill for under $500 with a creditor that ran late. (Actually it is a cell-phone bill where I co-signed for someone else and the someone else defaulted.) I started receiving calls from allied interstate about the 15th of the month. I must say that their representatives truly remind me of the south end of a north-bound horse! I called the original creditor a few days after Allied had called me, and made a partial payment by check over the phone, which was immediately deducted from my account. They said nothing about the account being in collections or anything, and only asked when I'd make my next payment, to which I responded, "In one month". They said "OK". I take this as payment arrangements being made with the original creditor.

When Allied Interstate called again, I related all of this to them, they said this didn't matter, and I needed to stop paying the original creditor and send payments to them. I told them that I would not do that unless the original creditor told me they didn't want my money and instructed me to only pay Allied Interstate. This seemed to anger the person from AI, and now I am receiving calls about every 3 hours.

On the last call, an hour or so ago, I asked the rep if they were the same company that was sued by Minnesota for illegal and improper debt collection practices. She said she didn't know what I was talking about. I told her that maybe she should check out the company she is working for, because if I filed suit against them, I would do all I could to drag her, personally, into it as well for the statements she was making. She told me to "hold on", and she did not come back on the line after about a minute, so I hung up.

The thing is, I asked this company for their mailing address and they wouldn't tell me. I asked for them to send me confirmation via USPS mail, with a clearly marked return address, and they just ignored the request, saying they needed to take care of this bill right now. Without an address, and with the many addresses listed on this site, how do I know who to send a letter to demanding proof they own the account? I don't see an address in Missouri. The phone number that the calls keep coming from is: 1-866-668-8638 and I could not find an address matching that number on the Internet.

My plan is to make these demands of them, and keep paying the original creditor. Hopefully, by the time Allied Interstate meets all the demands, the bill will be paid off anyway, unless the original creditor decides to stop taking my payments. With the bill paid, I could then tell AI to jump in the lake, or something perhaps more "suggestive" and in tune with the aggravation they have caused me. Is this a good way to handle the situation, or would someone suggest something better?

Thank you for any responses.