@Ranjith Reddy CSE use this javascript code with keyup event.........It will definitely help you........
function phone() {
code = "<%=txtphonenumber.ClientID %>";
if (!document.getElementById(code).value.match(/^[0-9]+$/) && document.getElementById(code).value != "") {
document.getElementById(code).value = "";
document.getElementById(code).focus();
alert("Please enter phone no. in digits");
return false;
}
}
all d best...