Where did you buy the phone from?
As far as I am aware, if you buy any of the phones linked with the offer for the £20 voucher either from an O2 shop or on the O2 shop online - www.o2.co.uk/shop, then you should have been given/sent a leaflet to claim your voucher.
If you have bought the phone from a third party such as Phones4U on O2 e.t.c then the offer won't apply, unless they are doing their own similar promotion, but in that event you would need to take the issue up with the third party company where you purchased the phone from.
