Indeed, trying to find signal when there is no signal or when it's too weak is big drain on the battery. So when the iPhone thinks there's no signal, it's going

Indeed, trying to find signal when there is no signal or when it's too weak is big drain on the battery. So when the iPhone thinks there's no signal, it's going to throttle its searches a bit to avoid draining too much power. In my experience, you can often get it to check again by locking the screen and then unlocking it again. Usually I only have to resort to going to airplane mode and back when the phone does have signal but it can't get reliable data (I'm assuming that going through airplane mode may cause it to switch towers).