I'm not as knowedgeable as Halket obviously is in these matters, but I'm in a very sticky situation myself with collection agencies. I've been doing a LOT of reading on what consumers can do where creditors and collection agencies are concerned.
First, collection agencies will use any tactics they think they can get away with, including some that are illegal, to get you to pay the full amount. One thing to note is that in many cases, the original creditor has sold your debt to the collection agency, usually for MUCH less than the original debt. I understand it can be as little as 10 cents on the dollar ! I don't know what happens if they get the full amount out of you when they only paid 10% for it, probably a big pay-day for them !
In any case, there are letters that you can write that will get results, forget calling them, you have no paper trail and no proof of anything (CYA !) Send all correspondence RRR too.
Two books that have been very helpful for me, you should be able to find them in your local library:
1 - "Debtors' Rights" by Gudrun Maria Nickel, Attorney at Law
2 - "The Complete Idiot's Guide to Beating Debt" by Steven Strauss and Azriela Jaffe.
My fiance wrote a letter from book #2 to a collection agency on a $7000 debt, offering a settlement. The agency accepted his settlement for LESS than 5% of that amount !! It does work, but if you offer a settlement, you have to have the cash amount to send them within 10 days of their acceptance. The collection agency never quibbled.. they just signed and returned his letter as he requested, siginifying their acceptance !