I think I can provide some useful advice. I used to work at a debt acquisition company in college. It's not exactly the same as a debt collector because a debt collector collects on behalf of another party while the company that I worked for actually purchased the debt from a 3rd party (banks, catalog companies, hospitals, etc.). They have the same general laws though.
First off, you need to determine if you legally owe the debt. If you think you do, then that's fine and you can skip to the next paragraph. If you're not sure you do, you have a legal right to have the debt collector validate the debt within a statutory time period, usually around 30 days. What this means is that they have to send you via mail evidence that you do indeed owe the debt and they're not just doing this to get extra cash. This responsibility is completely on them and when I worked for my company, people asked for this all the time and we had to supply it.
Next, once you decide to either pay it or not pay it, you also have a legal right to know if they are reporting it to any credit bureaus. They'll tell you either way. If they are and you want to dispute it, you have to let them know in writing and they'll usually suspend it from your credit report until it's resolved. If they don't, you won't have anything to worry about until it's resolved by either you paying it or them throwing it out.
Lastly, once you pay it or it gets thrown out, you also have a right to get it completely deleted from your credit history if it was ever on there to begin with. Getting it deleted is different than having it marked paid. Getting it deleted means someone checking your credit history will not even see that it was ever there. Make sure you insist on this and you can usually demand a letter as evidence of this.
Let me know if you have any specific questions and I'm sure you'll be fine. Also, if they ever cross any lines in terms of anything that can be deemed harassment, you'll be set with a nice lawsuit 🙂
Just a quick disclaimer, I'm not a lawyer or a CPA so any of this advice is just for your own personal knowledge and shouldn't be relied on for any legal matters. If you want actual legal advice, your best bet is to contact a licensed professional. I don't want to get sued 🙂