My iPhone is not taking my $15 itunes card

What do you mean by 'not taking' ? If you are getting an error message when trying to redeem it then what does it say ?

Was the iTunes card issued in the same country as your account is based (they are only valid in their country of issue) ? If it was and this page doesn't help then if it's something that only iTunes Support can help with (these are user-to-user forums) then you can contact them via this page (you will probably need to give them images of the front and back of the card, and possibly its receipt) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes