Much information about this is provided in the Caller ID FAQ here:
http://www.ainslie.org.uk/callerid/cli_faq.htm#Q_17
For example, this bit
Quote:
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With ACR, any call that has a Caller ID of "WITHHELD" is automatically ignored. Calls that present as "UNAVAILABLE", "INTERNATIONAL", "PAYPHONE" or whatever are allowed through. This is great for those suffering from nuisance calls of all kinds, less good for telesalesmen and other phone pests. However, "UNAVAILABLE" is presented by calls from the 0.5% of people on old UXD5 exchanges and (notoriously) on BT's FeatureNet service. This is BT's name for Centrex (delivered by a separate network of Nortel DMS100's - the small-business version, FeatureLine, uses the main network), which allows organisations to set up a virtual switchboard covering several sites within a town or further afield - it's often used by local government and universities. "UNAVAILABLE" also seems to be the default for some switchboards; companies are now finding that they have to configure their switchboards correctly if they are to communicate with all their customers. (I'm no expert on switchboards, but the impression I get from uk.telecom is that there is always a way round the "UNAVAILABLE" problem even if it has to be some kind of Presentation number like the main switchboard number rather than an extension.) However, recent reports that telcos can easily alter ISDN lines to present "UNAVAILABLE" are worrying....
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The law is available here:
http://www.opsi.gov.uk/si/si1999/19992093.htm#12
Quote:
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(4) Where presentation on the connected line of the identity of the calling line, before the establishment of a call, is available, the relevant telecommunications service provider shall ensure that the called subscriber has, as respects all or particular calls in the case of which such presentation has been prevented as mentioned in regulation 11(2) or (3), a simple means to reject the calls in question.
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I.E. telcos only have to provide ACR if the number has been actively witheld.