asp.net - How to set a server side validation for mobile number did not start with zero in asp.net 3.5?

Question

I have done it using javascript. But i have to do it on server side.

function Validate()    
    {
       var x = ctl00_txtMobileno.value;
       if (x.charAt(0)=="0" || x.length !=10 || isNaN(x)||x.indexOf(" ")!=-1)
       {
            alert("Enter correct 10 digit mobile number. It should not start with Zero !!");
            ctl00_txtMobileno.focus();
            return false;
       }

    }