PDL's in collections

I had 3 PDLs from online lenders while living in Ohio, now I'm in GA. To break the cycle, I closed my checking account and switched banks, mainly when I moved. Two of the PDLs have gone to collections. I've only ever received phone calls from those two collectors, have never received anything in writing. They, of course, were threatening (I committed bank fraud, yada yada yada) when they called; one I sent a debt validation letter to, have heard nothing from them since. The other I only received one call and was told to "consider myself served".

So my question is do I wait to receive something in writing before I begin making payments or, since I know the collectors and have their addresses (as given to me by the PDLs), should I begin making payments? If they haven't sent anything in writing within 5 days of their initial calls, aren't they in violation of FDCPA? Just not sure what to do since they've never sent me anything in writing.