Garageband doesn't recognize my guitar is plugged in

[ChildOfBelial]

I'm trying to plug my guitar into my iPhone 4's headphone jack using a standard 1/4" to 3.5mm adapter. Instead of it picking up the guitar, it still only picked up sounds from the phone's built in mic. When I hit one of the volume buttons, it says headphones. What do I need to do to get the phone to recognize that a guitar is plugged in and not headphones? I didn't want to have to buy one of the more expensive adapters since everything I've read online says a normal adapter should work.

Thanks.