Why do British Gas and npower both claim they supply us?

The two energy providers were even arguing about it in a conference callMy wife and I moved to our new home 18 months ago and there has been confusion over who supplies us with gas. Npower was supplying the property previously. We requested to switch to British Gas and were told there was a problem. Both npower and British Gas claim they supply us and over the past year both have sent demands for payment and debt collectors’ letters. There appears to be a phantom meter and no one can agree