BT aren't as mean as they could be. If you still have an account but can't pay BT will still permit 999 calls to go out, but that's it. Until they shut it down completely. Julie, if its an old number the providers rotate them so often that if its no longer in use it gets added to the list of new numbers for new customers, so its either dead, or the "that number has not been recognised" message will run. I strongly suspect its got a block like a payphone. Some boarding houses, hostels etc have payphones, so it could be in a location like that.
I hope this lady contacts you either way. Poor little baby.