Now a days it's common that every one has their mobile phone. I have a question that did you ever think why our phone number are 10 digit. Why can't it be 12 or 15 digit number? 

The answer to this question is, initially the people of telecommunication thought that as we have only 10 numbers we need to use them only. If we suppose 1 number to each individual then it is limited only to 10 people.

Next they thought to implement it with a 2 digit number but it was also limited to a few individuals only

Then the Department of Telecommunication thought to implement it with 9 digit number but it was also failed because our population is more than the required number of different phone numbers for each individual .

Then they finally decided upon and had the solution by introducing the 10 digit number. This would give us vast number of different phone numbers which would be efficient even for the next generations .

Now you may also have the doubt that initially our numbers used to start with 9 and now it is starting even with 6 right. At first they started numbers with 9 but when that series has completed then they moved to 8 and then to 7 and now to 6 and in future it may be 0 also.