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Many of the standard call plans from most companies are relatively expensive. Ntl's standard tarrif is much the same as Telewest, excepting only that the connection charge (the primary difference to BT, as far as I can see) is 5p on ntl and 6p on Telewest, wheras BT simply insist on a minimum call cost.
Telewest and ntl seem to have the call charges set to encourage bundled users of phone, TVand broadband. Thus standalone phone is pricey, but they do then offer the unlimited talk type packages, which become more competetive, if you are a frequent phone user.