Its the difference between a number being deliberately withheld, & a number being not available. All calls originating in the UK are tagged with a CLI code (Caller Line Ident). If the caller chooses to block this number being viewed by the receiving party, then Anonymous Caller Reject will stop the call. The majority of calls from outside the UK however, do not carry CLI codes, so ACR would also stop legitimate calls from getting through. So if there is no CLI on the call, it will get through.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV6), Talk Anytime Phone, VIVID 350, Virgin PAYG Mobile