Perhaps a silly one

In researching whether unpaid share capital could fall within s455 I came across the following AW post http://www.accountingweb.co.uk/anyanswers/unpaid-share-capital-s455-loan/479121 which discusses the issue. However I see from the posts that share capital that is not called up would not get debited to the directors loan account, which led me to the question of when is share capital called up and when is it not called up. Are subscriber shares automatically called up? If not, would this not result in  several instances of having nil share capital on the balance sheet? I know this is silly and I know the latter can't be true (well I've never seen it) but I just wanted to understand the rationale for my own sanity's sake.