An iTunes gift card isn't restricted to iTunes on the computer. You can also redeem an iTunes card on your iPhone. You can use your iTunes balance to buy music, videos and applications from iTunes and the App Store. As long as you have a valid iTunes card that has not been used and you have an iTunes account, you should have no trouble redeeming the card.
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Turn on your iPhone and tap the "iTunes" or "App Store" icon to launch either of the applications.
Touch the "Music" button if you launched the iTunes application, or touch "Featured" if you opened the App Store application. Both buttons are on the bottom bar of the applications.
Press "Redeem." Turn over your iTunes gift card. Enter the redemption code on the back of the iTunes card into the text box.
Touch "Redeem." Sign into your iTunes account. If you're already signed in, you will receive a confirmation that your iTunes card was successfully redeemed.