A good place to start is <a href="http://www.ispreview.co.uk" target="_blank">www.ispreview.co.uk</a>
A lot of 'freecall' ISPs use this system to stop you allowing your mates to get free internet calls using your username/password and some ISPs (eg Pipex) require you to have a compatible telephone exchange. I tried dialling from my parents house over Xmas and it rejected my call so they obviously do use CLI. They should always supply a 0845 number for use when away from home though.