How is it possible for, e.g. Tesco, to offer international call rates which equal or undercut B.T.'s rates ? In any case the initial route is over B.T's lines.

For that matter:  the inland voice call rate per minute is not trivial. But I can be connected to the internet - over my B.T. line - 24/7 for no more than the about £10 per month inclusive payment to AOL.