When will it end?

Customer: "Hi we sold our site in Stourbridge in February can you tell me if we have any contracts still live for that site?"
Me: "What contracts did you have? I know I could find out but it would be quicker if you knew what they was."
Customer: "I have an invoice for ADSL for the period March to May."
Me: "Ah OK you have Broadband let me look. Um it looks like you have not canceled the ADSL?"
Customer: "But we sold the site over three months ago now."
Me: "Did you tell us?"
Customer: "Well that's why I'm ringing"
Me: "You'll have to send us an email as we need written confirmation."
Customer: "Will we still get charged?"
Me: "Yes."
Customer: "But why? We sold the site in February!"
Me: "But you didn't tell us."

I suppose we're not classed as an integral part of their company? It's a bit like moving house and not telling the postman then complaining you're not getting any post.