Phone no validation

function ValidatetxtPhone(sender, args)
{

document.getElementById('<%# CustomPhoneNumber.ClientID %>').innerHTML = "";

var txt1 = document.getElementById('<%# txtPhone1.ClientID %>').value;

var txt2 = document.getElementById('<%# txtPhone2.ClientID %>').value;

var txt3 = document.getElementById('<%# txtPhone3.ClientID %>').value;

if ((txt1 == "000") && (txt2 == "000") && (txt3 == "0000")) {

document.getElementById('<%# CustomPhoneNumber.ClientID %>').innerHTML = "Please enter valid phone number";

return args.IsValid = false;

}

else if ((txt1 != "") && (txt1 == 3)) {

if ((txt2 != "") && (txt2 == 3)) {

if ((txt3 != "") && (txt3 == 4)) {

return args.IsValid = true;

}

}

}

else if ((txt1 == "") && (txt2 == "") && (txt3 == "")) {

document.getElementById('<%# CustomPhoneNumber.ClientID %>').innerHTML = "Please enter phone number(### ### ####)";

return args.IsValid = false;

}

else if ((txt1.length < 3) || (txt2.length < 3) || (txt3.length < 4)) {

document.getElementById('<%# CustomPhoneNumber.ClientID %>').innerHTML = "Please enter valid phone number";

return args.IsValid = false;

}

else {

return args.IsValid = true;

}

}