From aa-asterisk.org.uk wiki
Calling Line Identity. The mechanism by which the telephone network passes the caller's number to the callee usually via SS7. There are usually two numbers passed, (i) the caller ID and (ii) the billing number (sometimes known as the ANI in the United States). The caller ID (not the billing number) can then be displayed in one of several ways:
For those customers who have no other way of obtaining a callers number, PATS providers generally provide the number 1471 to read out the last caller's caller ID.
Analogue lines may make use of the Caller Display service which uses a special box to display the number whilst the phone is ringing. This service was introduced in 1998 and is defined by SIN 227, the first box of its type being the BT Caller Display 50. ISDN lines have this already have this facility built in, but your Communications Provider may require you to pay for the service first before they will transmit the data.
- CLIP - Calling Line Identity Presentation. On analogue lines, the "Caller Display" service, and the ISDN version of this service is known as CLIP. Other types of line (e.g. VoIP) also typically provide this service.
- CLIR - Calling Line Identity Restriction. On analogue lines, this is the "permanent withhold number" feature. On ISDN lines, this service is known as CLIR.
- COLP - Connected Line Identity Presentation. This service is not available on analogue lines. On ISDN lines and possibly some other types of line, COLP is the service where the number actually connected to is displayed on the phone when the call is connected. For example, if the caller dialled 0800 123 4567 and actually got connected to 01234 567890, the latter would be displayed on the phone's screen. This is one way that many of the geographic numbers on web sites such as saynoto0870 have been obtained.
- COLR - Connected Line Identity Restriction. This service is not available on analogue lines. The ability to withhold COLP information from the caller.
CLI can be marked as Withheld if the user does not wish to cause the CLI to be displayed at the callee's equipment. This is achieved by dialling 141 or setting the appropriate flag if on ISDN. Similary, the CLI can be released by dialling 1470. CLI can be marked as Unavailable if one or more exchanges along the route cannot handle CLI information. However there is very little excuse these days for this to happen and sometimes the flag is set when it shouldn't be. BT often flag calls as unavailable if they originate from non-EU destinations and they don't trust the CLI (to analogue lines, at least).