Ok so if you've ever had a cell phone you'll notice that there are some places that you don't get a signal. This is because cell phones are just that. They get a signal from a tower. This tower is a cell phone tower. And when your in the near by radius of one you can get signal. The radius that these towers give off will vary for each tower. And sometimes they will overlap each other. These radius's are known as cells, hence the word cell phone.
The above illustration shows and example of cells. The black dots in the middle of each circle are towers. The circles are how far the signal will go. The red means that the cells overlap on other towers signals. And the green says hey man your shit out of luck know cell phone using here.
So if your in a country type of area or a small town with no towers near by, like where I'm from then you SOL. Or maybe not. I have a wireless network and so do lots of other people. In fact more people have wireless networks then cell phones. In fact even stores and public locations have them and call them WiFi Hotspots. And WiFi is the secret to the cell phones of the future.
Anyone can easily set up a WiFi network. And it's very easy to do so. With a WiFi hotspot all you need is a WiFi enabled phone, with skype. With that and SkypeIn and SkypeOut you can send receive phone calls from and to other phones. You can also send SMS messages, Faxes, and much more.
Now this isn't full proof just yet due to wireless networks aren't always going to be in your area, but it is easier to set one up so it will catch on and be more reliable then cell phones ever have or will be. But as of right now cary both and only use that cell phone of yours when needed.