When was the first time you received a letter from them? If it has been more than 30 days then your options at this point are limited. You could send a debt validation letter to them but they do not have to stop collection activity including litigation when they get it.
At this point I suggest waiting it out and see what happens. It will do no good to pay it if you don't know what it's for. They will either sell it off to another company which will be great cause then you get another 30 days to dispute with the new company this time don't ignore.
The other thing they can do is file a lawsuit. Either they will just hire an attorney to sue you or the attorney will send you a dunning letter, that which you have yet a 30 day dispute window.
That's about the best I can tell you right now, Try to think really hard what this could be for. Is there any chance that it's really old, like more than 7 years? If so the SOL has expired, and also the Original creditor would probably not be on your credit report anymore. If the debt is time barred they cannot sue you, if they do you can have the case dismissed with prejudice and at the same time file a counter-claim against them for taking action that cannot legally be taken(FDCPA)
Hope this helps.