Why do we have to turn our cell phones off during air travel?

I have always wondered why we have to turn our cell phones off while we fly. I understand it is important during take-off and landing, but don't understand why. Also, a friend of mine told me we have to turn off our phones in mid-air because airlines don't want passengers annoying other passengers by talking loudly and constantly on their phones. Is this true, or is there another reason for us to turn them off? I've just been curious about this! Thanks!
They always say that you can turn your cell phone on in flight as long as you do not have it capable of recieving calls. Also you are able to turn on other electrical equipment in flight as well. So what is the deal with phones and being able to use them with calling?

You have to turn off electronic devices during take-off and landing (cell phones, wireless computer mouses, etc are banned for the duration of the flight) because they pose a risk called electromagnetic interference.

Electromagnetic interference is experienced by all of us on a regular basis. An example of this is if you put a cell phone near the computer, you can hear loud static in the computers speakers every time the phone rings, and the screen may start to shake. This technically should never happen, but the wire to each speaker is acting like an antenna, and it picks up side bands in the audible range. This is not a dire problem -- just a nuisance. But notice how common it is. In an airplane, the same phenomena can cause big trouble.

An airplane contains a number of radios for a variety of tasks. There is a radio that the pilots use to talk to ground control and air traffic control (ATC), a radio that the plane uses to disclose its position to ATC computers, there are radar units used for guidance and weather detection, and so on. All of these radios are transmitting and receiving information at specific frequencies. If someone were to turn on a cell phone, the cell phone would transmit with a great deal of power (up to 3 watts for a single phone). If it happens to create interference that overlaps with radio frequencies the plane is using, then messages between people or computers may be garbled. If one of the wires in the plane has damaged shielding, there is some possibility of the wire picking up the phone's signals just like a computers speakers do. That could create faulty messages between pieces of equipment within the plane. Now imagine what would happen if everyone on the plane were to use electronic devices, and you should be able to fully understand the ban on such devices.

Of course having a bunch of people on their cell phones at the same time would be annoying, and would make it harder for people to hear instructions should an emergency arise, thus it is another reason for the ban!

****EDIT****
The reason you can have it in "plane mode" during the flight (only if your phone has that capability) is so that you can listen to music or play games. This effectively shuts the phone down from trying to send and receive transmissions which could potentially interfere with the equipment on the plane. ******

Cell phones are NOT phones... they are low power WALKIE TALKIES and the have a TRANSMITTER that can interfer with the navigational equipment on the plane.

Yes the most basic part is that it can cause interference with instruments using radio frequencies. Plus even if you tried it is extremely rare to get signal at such high altittudes its been tested before.

First of all you must be blonde... You are not allowed to turn on your cellphone during the flight let alone on take off or landing mainly because the planes are using GPS to navigate and the cell signal at low altitudes might interfere with the flight instruments.

The airlines don't want any outside interference from sound waves....so you can watch that very boring instructional on how to use the seat belt :|

You cannot use you cell phone during a flight because you will not get any signal above 10000 ft anyway. You phone will constantly try to connect to a provider at no avail and will result in just wearing off your battery.
As for take off and landing, modern instruments do not get affected by the signal of your phone. The only problem is that it will interfere with communications (like when you have your phone next to a loudspeaker and it makes this loud and annoying sound, that's what the pilots get in their headphones and cannot hear clearly the instructions from air traffic controllers).