client side validation

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Javascript

Hi,
I want to validate a mobile no through client side. I have validated it as mobile no should start with country code and it should start with 9,8 or 7. Now i want to validate it so that it should not less than 12 digit including country code. how can i achieve this? can any one suggest me some sample code?
my code looks like this:

var txtPhone = document.getElementById(phoneID); //validating user phone no

     if (txtPhone.value != "") {

         var y = txtPhone.value;
         if (isNaN(y) || y.indexOf(" ") != -1) {
             alert("Invalid Mobile No.");
//             return false;
         }
         if (y.length > 12 || y.length < 12) {
             alert("Mobile No. should be entered with country code");
            // return false;
         }
         if (!(y.charAt(0) == "9" || y.charAt(0) == "8" || y.charAt(0) == "7")) {
             alert("Mobile No. should start with 9,8 or 7 ");
            // return false;
         }

     }