If a caller who owns a 0870 or 0845 number does not withhold their caller id, what will be broadcast – their geographical number or their 0870 / 0845 number?  Can they opt to choose what their caller id will display?

Also if a caller who owns a 0870 or 0845 withholds their geographical number when calling me is there a way to decipher their caller id?  I ask this question as some services that you phone seem to be able to decipher your caller id regardless of whether you withhold it or not.  In addition I have also heard of services advertising on the internet who decipher caller id for nuisance calls etc.  I have also heard of some internet answering services / mailboxes that are able to decipher caller id if you forward to them.

I would appreciate your comments on these questions

Thanks Dave

david151 wrote on Aug 25^th, 2006 at 4:17pm:

If a caller who owns a 0870 or 0845 number does not withhold their caller id, what will be broadcast – their geographical number or their 0870 / 0845 number?

If they haven't thought on then providing their number isn't withheld then it could display the geographical number.  It can, however, still be their 084x/087x number or even withheld (mostly withheld I've found).

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Can they opt to choose what their caller id will display?

They can and I don't even think its regulated to prevent misuse.  Having this facility though would cost them a lot which is why I think most of the time they just choose to withhold it.


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Also if a caller who owns a 0870 or 0845 withholds their geographical number when calling me is there a way to decipher their caller id?

Not as far as I'm aware.  It may be possible if the number is simply withheld to display the true geographical but if the company has chosen to display their 084x/087x number then I don't think its possible because it would be the likes of BT, etc that would then set the caller ID before the call even leaves their exchange.

The reason why I think it is different if the caller ID is simply withheld is because I think the Caller ID is actually still sent but it is flagged as being withheld and caller IDs are meant to respect this and therefore not display the number.

I say this because I know a friend who has cordless phones that allow you to set a name to a withheld number and their cordless phones are still able to determine that despite when I ring the number is definitely withheld, their cordless phones displays that the number is withheld but has my name on it (because it was setup that way).  When someone whose name isn't saved rings with their number withheld then their cordless phone simply displays withheld.  

Therefore my number must be transmitted along as normal but its flagged as private and therefore my friends cordless phone (after they setup my name after I've rung them) sees my number but also the fact that I've asked for it to be withheld so instead displays withheld but also my name (because I'm saved).  If that makes sense?

bbb_uk wrote on Aug 25^th, 2006 at 5:42pm:

Therefore my number must be transmitted along as normal but its flagged as private and therefore my friends cordless phone (after they setup my name after I've rung them) sees my number but also the fact that I've asked for it to be withheld so instead displays withheld but also my name (because I'm saved).  If that makes sense?

Interesting... it would be useful if someone in the telecom industry could confirm this.

When you dial 1471 on BT, sometimes is says, "The caller withheld their number." But sometimes is says, "We do not have the caller's number to return the call," for example for international calls. Perhaps this is the distinction between those transmitted but flagged as "withheld", and those actually without the information. Which does it say after an 0870 company has called?

Quote from david151 on 25. Aug 2006 at 12:17:


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Can they opt to choose what their caller id will display?

I think that any answer to this question should realise how weak the security surrounding caller ID has become. There is a story doing the rounds here in the US that a small fake Caller ID seller is accusing Paris Hilton of hacking into voicemail accounts on an un-named mobile phone network. Apparently, you can buy a card that allows you to determine what caller ID you want the called person to see.

For details see: http://www.infoworld.com/article/06/08/25/HNhiltonhacking_1.html?source=NLC-SEC2006-08-28

and I quote from that article:


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The scandal illustrates how the telephone industry has been affected by inexpensive telephony software, like the open-source Asterisk telephone system. Recently phishers have been using this software to set up inexpensive phone networks that give their fake e-mails an added air of authenticity, for example. And with less than 10 employees, SpoofCard.com was able to use Asterisk and Linux to create a line of business that would have been far too expensive just ten years ago. The fake Caller ID vendor sells US$10, 60-minute calling cards that let users call a toll-free number and type in whatever Caller ID number they want their call to display. While SpoofCard.com maintains there are legitimate uses for Caller ID spoofing -- allowing remote employees to appear as though they are dialing from their company's phone system, for example -- the latest incident indicates that this technology has created new opportunities for misuse as well.

It's a worrying thought!

I also understand that the telco always know where a call comes from. They need this information for billing purposes. It can also be used by emergency services. However, they will not generally release it.

I think the bottom line is that you really can no longer trust what your telephone is telling you!

It also allows you to speak normally and  change your voice into a womans or mans voice .

See: http://www.spoofcard.com/faq.asp for full details.

jrawle wrote on Aug 25^th, 2006 at 9:43pm:

bbb_uk wrote on Aug 25th, 2006 at 5:42pm: Therefore my number must be transmitted along as normal but its flagged as private and therefore my friends cordless phone (after they setup my name after I've rung them) sees my number but also the fact that I've asked for it to be withheld so instead displays withheld but also my name (because I'm saved).  If that makes sense? Interesting... it would be useful if someone in the telecom industry could confirm this...

I managed to get the following info which confirms that our number is actually transmitted along just that a "flag" is set which tells the CLI display to display "withheld" instead.


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The situation is that when you withhold your CLI, it results in a flag being set in the network signalling to indicate this.  Within the UK/EU*, the CLI is still sent, but based upon it the terminating network won't release the CLI to the called customer.  For calls leaving the EU, the CLI is masked when withheld because we can't be sure whether the far end will respect it & have no legal recourse if they don't. *By EU I mean "countries with equivalent legislation to the EU"...so I'd include e.g. Switzerland, Norway and Australia, but emphatically not USA.

Based on the above then that explains why some calls, if routed outside the UK (ie via call18866, etc), is sometimes withheld and why international calls are withheld.

Unfortunately I don't have the source, but I do remember reading on a UK site that they offered the service, for £60 per month, of dictating what number showed up at the other end.

So perhaps those numbers showing as 070....... are actually from landlines.

An interesting read
http://www.theregister.co.uk/2004/07/07/hackers_gut_voip/

and http://www.alternet.org/columnists/story/19295/
But in the meantime, everybody else is coming out to play! A phreaker calling himself Lucky 225 has a nifty little device that allows him to use VoIP to spoof phone numbers. He can make caller ID programs think he's calling from anywhere; he can also unmask the phone numbers people try to hide with caller ID blocking. One use for such a device, aside from personal amusement, is to activate someone else's credit card. Since most credit card companies authenticate your identity by requiring you to call from your home number to activate a card, Lucky's trick could turn out to be quite tricky indeed.

Some more useful info here

:http://news.bbc.co.uk/2/hi/technology/4482139.stm

Also worth a look at

:http://codegods.net/cidmage/cidpad.htm

What a shame that Oftel refused to insist on a marker for all numbers displayed that were not the real number.
And I mean SHAME. Too little, too late, too bad...

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Can they opt to choose what their caller id will display?

BT offer a Presentation Number service click here for more info.

:( So I guess, we can never know for certain where someone is actually calling from now! [smiley=thumbdown.gif]

The more sophisticated ISDN terminal equipment allow you to program your "caller ID"