Q: iPad code redeemed on iPhone

Does it not appear in the Purchased tab in the App Store app on your iPad for re-downloading ?

Re-downloading : http://support.apple.com/kb/HT2519

I haven't downloaded anything yet.  I want the iTunes store to recognize my redeem code on my iPhone. It won't.

What happens when you try enter the code ? Where has the code come from, a gift card, emailed gift, DVD/Blu-Ray, and was it issued in your country ? If you are getting an error message then what does it say ?

iTunes says the code is already used. I didn't use it to buy anything. iTunes says I do not have any credit.

You've tried logging out of your account on your phone by tapping on your id in Settings > iTunes & App Store and then logging back in ?