If you are receiving calls from "unknown numbers" via caller id and..
it is debt collector.....if you phone has the set up option, block all "anonymous" or "unknown number" calls.
That way they must disable it on their end in order to call you and if they hang up on you, you can google their phone number by what shows up on the caller id pretty simply.
If they want to find you that bad and they have your phone number, they must have your address. Let them put something in writing and send it to you via United States Postal Service. That way what you have is in writing and so is your response for proof of any debt.
No bottom feeding scum bags do anything in writing, they ALL do it behind "unknown number" caller id's and by breaking the boundaries for FDCPA, not that violating those "guidelines" mean anything either.......when you file complaint with FTC about violation, nothing happens and debt collectors continue harassment BUT DISABLING their ability to harass you by calling you and blocking THEIR number means you can then......in turn.....tie up their lines.....see how to "block" your number when calling them back.
Let them see how it feels to have your day interrupted repeatedly by hangup calls and being uable to get what they loosely call 'work' done.