if user dont follow validation in javascript then textboxs should remain focus

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Javascript

what i want is when user breaks my javascript validation he should not be allowed to move further until he corrects his mistake but my javascript code is allowing him no matters he is not following my validatons . here is my code:
function phval() {
var a;
a = document.getElementById("TextBox5").value;
if (a.length != 10)
{
alert("Enter 10digit number");
return false;
}
else
if (a.charCodeAt(0) == 57 && (a.charCodeAt(1) == 56 || a.charCodeAt(1) == 55) )
{
alert("valid Number ");
return true;
}
else {
alert("Your Number Should Start with either 97 or 98");
return false;

}
}

it works in way that if user enters less than 10 digit in phone no. field alerts, but clicking ok, user is able to all things. what i want is if user number starting is not 97,98 and length is less than 10 then he should not allowed to carry on until and unless he corrects . what can i do.