There are only a handful of UK-compatible caller-id modems for analogue lines; there have been some reports of success with courier modems but that is inconsistent. The most reliable ready built alternative is the Meteor unit, but it is expensive for what it does. A number of people have had success modifying conventional caller id units to provide a serial output. These apparently work fine with my homeseer plugin (and presumably other software which follows the BT format for caller id.
To further complicate life, there are two caller id standards prevalent in the UK. All BT exchanges use the BT standard which transmits the caller id as a burst of data before the first ring. This was intended to allow for activities such as remote meter reading, without the need to disturb the householder at all.
The cable operator franchises are divided between those that don't offer it at all, those that offer the US Bell standard, where data is sent after the first ring but before the second ring, and those that follow the BT standard. This mess is the result of the repeated merger of different franchises: it's now pretty much at the point where you it's impossible to tell until you actually order the service whether it is available.
With the exception of the meteor, which I believe works with both Bell and BT standards, most kit will only do one standard or the other.
ISDN users can also have CLI delivered over the D channel (signalling channel), as a premimum service. This will include the number that the caller dialled if MSN numbering is activated on the line. The last time I investigated this issue, CLI delivery over the digital channel was not available for Home Highway users. I have had conflicting reports on availability for Business Highway users. The ISDN feature set is extensive, and offers some great features, and the cost of terminal adapters has fallen dramatically in the last 18 months or so, making the choice of ISDN 2e over Business Highway attractive. Small ISDN PBX's can be imported from Germany and represent excellent value; Siemens also manufacture a fully featured ISDN compatible DECT base station. *LINK*.
CLI operates normally over an ADSL enabled analogue line. (ADSL cannot currently be delivered over an ISDN circuit in the UK).
CLI & Comfort
CLI is not passed consistently to extensions by a Comfort system when the system is armed. This is exceptionally irritating because CLI is especially useful to note missed calls when out and about. The only workaround is to either (a) wire the extensions in parallel with Comfort's tel-in, losing the ability to interact with Comfort via an extension, or (b) connect a separate caller id box (or meteor unit) in parallel with Comfort's tel-in and place this is a strategic location.
Automatic routing of calls that have had the number withheld (perhaps to an answering machine) can be achieved by using a change over relay to switch all three wires used by the extension circuits in parallel. This kind of functionality can be readily achieved using a xAP PIC relay and xAP caller id interface. It's also worth noting that BT and some cable companies are able to offer Anonymous Caller Rejection (ACR) on a limited number of exchanges.