Andrews & Arnold Ltd
BRACKNELL
Calling Line Identity means that you know who called you, or rather the phone number of the telephone line that called you (not quite the same!).
It means that when you make a call, your number is passed through the phone network to the person you are calling. What they do with this information is pretty much up to them (subject to normal data protection laws, etc).
Obviously, some people don't want their number to be sent out whenever they make a call - read on...
You can assume your number is sent unless you take action to stop it. This is not 100% true, as some telephone companies can't send numbers like this, but most can, even when using indirect carriers or mobile phones.
If you want to stop your number being sent, you can do one of three things :-
Note that your number is always sent through the telephone network regardless. It is simply the display to the end user that is withheld. (i.e. you cannot hide your number from the authorities or the telephone companies). Remember that some people may automatically reject withheld callers.
You need to be able to release your number sometimes, particularly when calling someone with Anonymous Call Rejection.
You can normally do this. Again this is done by dialing a code in front of the number you are calling, usually 1470.
Not all phone companies have this feature yet. If you don't then you should complain as you will not be able to call people with ACR.
Calling the BT test number 17070 will tell you your CLI if it received it. Try 14117070, 147017070, etc. This is a BT test number so don't abuse it.
There is a service called Anonymous Call Rejection or ACR. This allows you to set your phone to reject any Withheld calls. It will still allow Unavailable calls, but if someone withholds their number then they will just get a message when they call. This is a legal requirement on telephone companies, but it is not required to be free of charge.
See Statutory Instrument 1999 No. 2093 for details of the law on CLI.
If you have ISDN you may be able to set your phone system or computer to automatically reject such calls.
Some companies (indirect carriers and some mobiles) incorrectly force your CLI to be withheld - this is against OFTEL rules and you should complain even more now that you may not be able to call people with ACR enabled.
Mobile operators are still not complying with the regulations and allowing anonymous call reject. OFCOM (OFTEL as was) were no help. We got as far as the Information Commissioners Office agreeing :-
What is the point?!?! The key issue here is that the regulations say the subscriber must have simple means to stop witheld calls. The user does have such means - pressing the red button when the see a witheld call, but the subscriber does not, and that is what the regulations require. The only sensible way for them to do this is to offer an anonymous call rejection server (like BT do). None of them offer such a service. Crazy!
If your phone company has a Caller Displayservice, such as most BT exchanges, then you can get a display box or phone that shows numbers before you answer the call. In the case of digital mobile phones the number is normally displayed, and possibly even a name (looked up from the number in the phone's memory).
In the case of a land line the number is sent to the display unit when the call comes in. There are however two different systems for this - a BT/UK system and an American system. There is also ISDN which is different yet again and not the same as either of these systems.
If you want such a device, make sure the person selling it is aware where you will be using it (which phone company), so as to ensure you get a compatible unit. Some phones/displays work with both commonly used systems.
For BT the basic unit is called a CD-30 (there are later versions now) and costs around £20 to £50(inc VAT). This shows the calling number when the phone rings, and also remembers the numbers and date/time of the last 30 calls (even if you did not answer them). There are many phones that incorporate the caller display facility.
Obviously, if you have a display device (as above) it is likely to show the details of the last caller. However, phone companies providing CLI usually have a service that reports the number of the last caller.
On BT this is free to most customers. You find the number of the last caller by dialing 1471 . You hear the number and time of the call, or a message saying the number was withheld or unavailable.
On BT you can call back the last number by dialing a 3. e.g. dial 1471-3. Note that this puts the call through BT and not any anternative carrier you may normally use via a prefix.
Indirect carriers used to be accessed from a BT line by dialing a prefix, such as 131 <PIN> or 132 (e.g. Cable & Wireless). These days you can pre-select a carrier and it is used without a prefix. Either way, the carrier sends CLI with calls over their network. If you use a prefix and you want to use 141 or 1470 then you dial them in-front of the 131 <PIN> or 132 code.
E.g. if you have C&W 132 and want to call 01344400000 over C&W, but withholding you number, then dial 141 132 01344400000.
Generally incoming international calls do not show a number. On a caller display box they may show as "INTERNATIONAL" in the main part of the display. This is changing, and increasingly you will see European numbers being displayed.
Outgoing calls to some countries are now showing calling number from some lines and some types of calls. e.g. ISDN data calls tend to be the first to be present calling line identity as this is often used for security for dialup ISDN data connections between routers.
There are a couple of quirks with ISDN and calls made from DDI extensions on an ISDN PABX. Basically, if you have a block of say 10 numbers, and call from of of these, then the direct dial number is presented from BT ISDN lines to other BT customers allowing a direct call back to your extension.
It seems however that some companies don't interconnect with BT on this very well, and they present the base number (ending in 0) rather than the direct dial extension number, or no CLI at all. See the ISDN interworking page for more information.
ISDN also has a system for marking the number withheld without using 141. How this works depends on your telephone system.
Some people show as withheld but they said they did not hide their number? Yes, some of the telephone networks are operating in breach of the OFTEL code of practice on calling line identity and sending Withheld for all calls. Known culprets for this are in as follows. If any of these are wrong, please let me know - times change.
If you use any telco or carrier that does this then let us know, and please feel free to complain to OFTEL as much as possible. If we have it wrong, let us know as well.
There is a service called Presentation Number. It costs money, but allows you to present a different number when you call. The number is pre-vetted by your telco to confirm it is yours. It is typically used to present a free (e.g. 0800 type number) or low cost (e.g. 0845 number). It cannot be used to present a premium rate number. It could be the number of your head office, for example.
Sadly, on ISDN, BT have seriously missed the plot. The sensible thing to do would have been to add one or more pre-approved presentation numbers to the exchange so that the ISDN equipment can quote them as calling number, and it be passed on. This would allow a call centre to choose to send a DDI, or a main number, or one of the presentation numbers on a per call basis. Sadly, on ISDN, if you have presentation number then that is the one number you can send - you cannot even sent DDI numbers any more, and you can only have one number. Crazy. We did tell BT when they were developing it, but they ignored us. What can you do!.
However, this does mean that there is really no excuse for people sending witheld any more. They could set up presentation numbers as they require instead.
Just like the dialing codes for mobiles, and other services, OFTEL regulate the dialing prefixes (starting with a 1) and have defined the codes used for calling line identity control. If a telephone company offers the feature then they must use these codes for it :-
| 141 | This is a prefix to Withhold your number |
| 1470 | This is a prefix to Release your number if you normally Withhold it |
| 1471 | This is a number than tells you the details of the last calling number. |