Some numbers (overseas call centres especially) have numbers that are 'unavailable' and not withheld.
Unfortunately ACR doesn't block these types of calls, it only blocks calls that are actually withheld. As Russ said CS can add this on free of charge for you.
If you are getting calls which are silent for a few seconds and then the line goes dead it's probably a call centre of some description. You can register with the telephone preference service which should stop these by going to
www.tpsonline.org.uk or dialling 0845 07 007 07