Answer 1

Suppose you need to get a random no from 7,8 or 9 into your phone number as the first digit

So the best way will be to store the number in an array.

int no[10];//C++

And randomly determine every number.

Using rand();//C++ code

And to limit rand() to a range, use the following format-:
(rand()%U)+L;
//U=Upper Limit
//L=Lower Limit

And here it will be -:
(rand()%9)+7;