I used to work in debt collection. I know a bit about the industry, at least the way it is practiced in Europe. And I have been pursued by debt collectors in the USA.
There is never a time when it is advisable or a good idea to speak to a debt collector. The system is complex, and completely corrupt. Every word you say to them gives them more information and leverage.
If it ever comes down to it, either pay the debt off completely, or have every communication go through a lawyer.
You will never win against a debt collector. Ever.
You will win if it goes through a lawyer. I had to do that two years ago for a debt that was sold without any communication provided to me prior to seeing a credit hit and call from the collection agency. After sending out some legal requests that situation went away very, very quick.
Debt collection is scum.