Once had a call from 'microsoft' to say my computer was at risk. (yeah right). The caller sounded indian..... Put the phone down and dialled 1471. The number was 123456789. Mmmm.
More seriously, it's a blocked number, probably a telemarketer, collection agency or someone else who doesn't want you to know who they are. It could also be one of those numbers that lifts large amounts off you when you respond. Good you didn't.
Or is it tonyav?
If you call your local telephone provider and ask them to add "Anonymous Caller Rejection". Then these people will get a message saying that you do not accept calls from unidentified callers.