How to Rotate an iPhone Screen When Texting
Several users have complained that the Messages app fails to change orientation after upgrading to iOS 7. This effect is the result of iOS 7 having its Orientation Lock engaged by default on some iPhones, such that rotating the phone does not have the intended result. Even if you're familiar with the Rotation Lock in previous versions of iOS, version 7 places the lock in a new location.
Open the Messages app and begin composing a new message.
Drag your finger up from the bottom edge of the iPhone to reveal the Control Center.
Tap the "Orientation Lock" icon, which appears as a circular arrow with a center padlock, to toggle the lock on or off.
Swipe down on the Control Center to close it. Even with the Orientation Lock off, the screen won't rotate with the Control Center open.
Rotate your iPhone to the desired orientation. The screen should rotate to compensate. If you want to lock the screen in the new orientation, tap the "Orientation Lock" icon again.