My view is it's best not to call them back; screen your calls. If you do, expect more aggressive collection actions. However, if you feel the need to call, you can tell them to stop calling. Doesn't mean they will stop though. Often when one ignores such "zombie" debt collectors they go away. They paid little to nothing for the debt (assuming they even own it to begin with) and will move on to easier pickins who will respond.
Zombie debt is very old debt that is both out of statute of limitations (4-6 years in many states, but can be little as 3 to upwards of 10 in a few) and also upwards of 7+ years old, and no longer reportable to the credit bureaus. Statute of limitations (SOL) and length of time a creditor / debt collector can report are often different. For example in Pennsylvania, SOL is 4 years, but credit bureaus can report delinquent debt for ~7 years. When both SOL and credit reporting time are both exceeded, that's zombie debt.
A cease and decist letter can be effective, but you'll need to find out who the zombie debt collector really is - they often lie on the phone and/or evade the question. Also, they can transfer the debt, and the cycle begins all over again.
Unpaid debt can generally be pursued indefinitely, but there's a limit to the length of time (statute of limitations; SOL) a creditor / debt collector can utilize the courts to collect on the debt. Again, to reiterate, the debt itself remains a legal debt indefinitely and can be pursued. However, the vast, vast majority of creditors / debt collectors will reduce, and eventually cease, collection activities sometime after SOL and/or ~7 year credit reporting period has elapsed.
