Although recording a phone conversation without the other party's consent is illegal in some states, it can be done if you are conducting an interview and have informed the other caller that he is being recorded. While the iPhone doesn't have default feature that allows you to record a conversation, there is a third-party recording application that can be purchased from the App Store. To successfully record a conversation, you must purchase recording time and place a call while in the application.
Press the "Home" button on your iPhone and tap the "App Store" icon. Tap the "Search" button at the bottom of the screen, and tap the search bar.
Type "Recorder" and tap the "Search" button in the lower-right corner. Tap the first search result.
Tap the "$0.99" button, and tap "BUY." Enter your iTunes account password, and click "OK."
Tap the "Recorder" icon on your menu screen and tap the "Call Recording" button at the bottom of the screen.
Tap either "1 hour of record time" or "8 hours of record time" and click the "Buy" button to confirm the purchase.
Tap the "Call" button and select a contact from your address book, or tap the keypad button to manually enter a phone number.