NGN - gradual
0845, 0870, 0871, 0808 and 0800 numbers can be
gradually introduced to your customer base with no sudden changes. Existing
customers can still reach you either on your normal geographic code (eg 020) or
on your non-geographic code (0800). Your aim should be to eventually have
everyone calling on your non-geographical number so that notification of future
number changes becomes unnecessary.