how to validate sri lanka phone number in asp.net

You can use from the ASP.Net Validators available. You can make use of RegularExpression validator or may be custom validator for this purpose.

Also take a look at MaskedTextBox from the AjaxToolkit if that suits you.

Alternative: You can also do it using javascript. call following javascript function on button click.

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")
  {
      alert("it should start with 9 or 2 ");
      return false;
  }
}

for more detail check, JavaScript: A Quick Check[^]
Validate Phone Numbers: A Detailed Guide[^]