10 digit number

Why do we have a 10 Digit Mobile Number?

Since the time I am using a mobile phone, we have always been getting a 10 Digit Mobile Number. It’s been so many years but we still have the same number of digits – 10.
Initially, the numbers started with ‘9’. Probably if you are using the same number for the last 7-8 years, you might still be having a mobile number that starts with 9.
As time passed, we got numbers starting with 8, 7 then 6. So, what is the exact reason behind these 10 digits?