IPhone 3gs won't turn sideways when texting?

The lock orientation is NOT on. I've disable and enabled it many times, to try and make. All of the apps and photos turn fine when I tilt my phone. Literally, ONLY the texting won't turn sideways. Can anyone help me figure this out>?

It maybe that specific app. Some apps will only allow the device to be a certain way. If it is apple's messaging app, try restoring it through itunes.