Returned equipment gone missing

I have recently had to leave virgin media as I have moved house and the new property doesn't have the service there to move over. As I was not living in the property I gave the router equipment back to a virgin engineer that was doing an installation on the street at the time. He said they will be able to track the return with the serial numbers on the equipment etc...

I rang up to notify yourselves what had happened and an agent told me thank you for informing them and this had been documented. Fast forward 2 weeks later I am now getting charged £200 for failing to return anything. I am left on hold for over an hour then the line just cuts off, and on live chat I'm told I'm just about to speak to an agent then it kicks me off. 5 times I've been waiting to speak to an agent this morning then the chat just kicks me back to the sign in page. I have exhausted every possible avenue to the point of this almost being a scam now! How am I supposed to sort anything out when I can't even speak to anybody again about what's happened??

Absolute shambles