Harassment by Ampath debt collectors for unspecified amounts

Once more I'm being badgered by Ampath's debt collectors claiming I owe money. Ampath has not sent me an invoice, nor have they sent me a statement. The debt collectors want me to give them my personal details on the phone before giving me any details. I am not prepared to give out details when they are the ones phoning me. So here we are again, for the umpteenth time. This harassment must be borderline *******. The directors of this company should be ashamed of themselves.