Also when you are out of range your battery usually drains faster. Part of this is due to some phones falling back on an older technology such as analog which takes more power and partly because the cell phone will 'ping' more often trying to find a tower. You may need to power off to conserve energy.
We stopped buying analog capable phones this last round as the analog network is slowly going away. A tower near my parents must have been taken offline sometime last summer as we stopped getting a signal there unless we go half way up the mountain. Our phones would constantly go dead then trying to find a signal.
Also text messaging, as others have stated, takes less time, just a fraction of a second is all it needs and a text message wil stay queued up and send when the phone does find a signal enev if only for a brief time. I made sure our phones are text capable enev though we get charged for them since we don't have text built into our plan, in an emergancy the cost is a small price to pay.