In my darkest days, before I got my financial act together, I was being pursued by debt collectors. It seemed really scary until I educated myself about debt and then it all became very simple. I did not give any comment to anyone who contacted me by phone and, in fact, changed my phone number so they couldn't find me. Then, I got the names of the debt collection agencies from my credit report and wrote to them asking for letters of authentication. If they did not authenticate the debt, then I simply ignored it until it fell off my credit report. If they did authenticate it, then I wrote back to them with letters that did not admit that I owed the debt or offer payment, but instead asked for a deal to settle the debt for a reduced amount in return for deletion of the negative account from my credit report. If they responded affirmatively, I paid the settlement amount. If they responded negatively, then I ignored the debt until it fell off my credit report.
The most important thing is to not admit the debt is yours to the collection agency or make any comment agreeing to pay for it, because that will reset the seven years before the negative account will fall off your credit report. Even if you repay the debt to the collection agency, they are not obliged to remove the negative account from your credit report, unless you have a pay for deletion agreement in writing from them. If the debt is really old past the statute of limitations and it is no longer on your credit report, then just forget about it. Some people say that it will pop up again a a problem when you go for a mortgage, but that didn't happen in my case. Nobody can do anything to you about it once it is past the statute of limitations, so the debt may as well not exist anymore.