They could possibly have done the same as I do on occasions (usually with succeess) and that's pick a group of numbers and call them. Just like spammers, when they get an answer they know they have a live one. If it's a company or private, they may get business out of the call. Even if they don't get an answer, there's plenty more to try - it's probably an auto-dialer on a computer system.
You've now confirmed your number is live so unless you shut them down it will likely be passed on as live to the main database, which may be sold on. Emailing them may not be successful either as they may not give a damn. You may have to take the complaint elsewhere.