How many cell phones are there per person in the world?

6/7 cell phones for persons. (1 for person)

What are the differences in different countries? Can you comment some specific cases?

There are some that have over 100%.

What will happen in the future?

 There will be more phones than people in the world. 

How is it that we can speak on a mobile phone without hearing other conversations from other people

Because differents methods are used.