How to validate phone Number in asp.net properly?

function Validate()
{
  var x = document.getElementById("txtPhomenumber").value;
  if(isNaN(x)|| x.indexOf(" ")!=-1)
  {
     alert("Enter numeric value");
     return false;
  }
  if (x.length > 10 || x.length > 8 )
  {
     alert("enter 10 or 8 characters");
     return false;
  }
  if (x.charAt(0)!="9" || x.charAt(0)!="2")
  {
      alert("it should start with 9 or 2 ");
      return false;
  }
}