I'm not certain on this ben but the 30 day limitation is applied to how long the voucher lasts for, not how long the credit stays on the phone for for.
i.e. a voucher of £15 is purchased and you don't ring up to credit your account with the voucher within 30 days, you've lost that £15. :-(
i.e.2 a voucher of £15 is purchased and you top up your phone within 30 days with the voucher number, you get your credit and it lasts until you've used it.
hth and i may be wrong, JT