A mobile phone has many different functions and new functions are being created for it all the time, some of the functions that it can already perform are: Call people just about any where in the world from just about anywhere in the world, send text messages just the same as call's, get information from the internet and use in-built calculators to do some simple math.
A cell phone transmits information via digital wavelengths, not by Amplitude Modulation or Frequency Modulation, however it's radio signal that it transmits the audio on is very close to being FM.
The Mobile phones signal is transmitted all around the world because it travels though a range of Base stations which are a bunch of antennas which will receive the signal and send it onto either the next base station along the way or to the device you are trying to get in contact with.
The digital mobile phone frequency is between 890 and 960 MHz. While a PCS (Personal Communications Services) phones (which are completely digital phones) operate between the frequencies of 1.85 and 2.15GHz, however PCS phones require more Base towers and to have them closer together then Digital phones but PCS phones have a much clearer sounding signal.
http://www.howstuffworks.com/cell-phone.htm used on 7/04/08
http://au.answers.yahoo.com/answers2/frontend.php/question?qid=20060831083339AAH415i used on 7/04/08, this source isn't as reliable as others since this is a Q&A site.
http://www.science.org.au/nova/022/022print.htm#radio%20waves used on 8/04/08.
http://electronics.howstuffworks.com/question31.htm used on 8/04/08.
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