I had a similar thing a few years ago when my house was let to tenants whilst I lived abroad.
I came back to the UK and needed to speak to the new tenants who had a problem with a failing built-in freezer. I rang my "home" telephone number on my mobile and had a conversation with the tenant, no problem.
However, I rang again a week later, only to get a "number unobtainable" tone. I got BT to investigate and they advised me that my previous tenant, on moving out over a year before, had cancelled the line (and nicked the phone, which I owned). A new number had been put on the line when the new tenant moved in, at his expense.
What was and still is, completely unexplained, is that I had no idea about this and the original number worked in the first instance! BT said it wasn't theoretically possible, but it certainly happened.