Do as suggested by Postdog, above. Before that and above all don't pay or offer to pay anything. If you don't owe anything, you don't owe anything. If you made an offer you could be held to have agreed that you owed something and then they'd sue for the whole sum plus interest
They can't get money from anyone simply because they are debt collectors. Whoever they are, they have to go to court and sue you first (they won't do that without you knowing, because they have your address and that would be the address the court uses) and get a judgment. The papers are all sent out by the court and you deal with the court office. You send back a form they supply (your defence) in which you say you don't owe anything and the bills have nothing to do with you and explain why. That should be the end of it. The claimant debt agency or the company they are acting for has to prove the opposite, that it was you, it is your bill, you did have that phone etc etc and that they sued within six years of the debt. When they discover they can't, they give up.