Under the old talktalk packages (e.g. Talk International Anytime) caller ID was a cost option at £1.50 per month. - UPDATE - Apparently the price has just gone up to £2.50 per month!!
If you are paying for this save yourself some money and log onto yur account, navigate to Manage my Services, deselect caller ID and replace it with the free Privacy Pack.
I double checked this was not an error with customer services who confirmed that this feature is now free but you have to request it (grrr!)
Not much good if you have a multi package of features, but if you have one or two it's worth 5 minutes of your time.