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Most home phones don't have CLID, mostly because the phone companies charge extra for it.



All mobile phones do.



Most office phones do, but on some this is imited to internal CLID only.



For a private UK number, you will generally see the number in domestic form (no +44, no digit grouping), unless the caller has requested their CLID be withheld, either permanently (through the telco, through mobile phone settings, or by prefixing the call with 141). All telcos (BT backbone, cable or mobile) have CLIDs sent inter-operator, but there are still some oddities, like Kingston Communications, who have always had local loop rights in Hull. Where the number is not sent, the 'unavailable' message depends on the receiving phone.



For a call from wthin a UK switchboard, you may see the direct line (if applicable), the switchboard number, or nothing, formatted as above, depending on switchboard type and configuration.



For international calls, what you see depends on the inter-operator agreement. Some other countries' systems send a compatible CLID, in which case you'll get the number, ungrouped, with the +xx prefix. If not you may get 'international call' or a standard number unavailable message.