Executing VB Via JavaScript

I am using an ASP.NET AJAX control (ValidatorCallout) that requires
client-side validation to work with a custom validator I added. This
is an example of some code that works:

<asp:CustomValidator ID="CV_PartNumberExists" runat="server"

ClientValidationFunction="CheckPrime"
ControlToValidate="PartNumberText" ErrorMessage="b><br />A Part
Number is required."></asp:CustomValidator>
<script language="JavaScript">
<!--
function CheckPrime(sender, args)
{
var iPrime = parseInt(args.Value);
var iSqrt = parseInt(Math.sqrt(iPrime));

for (var iLoop=2; iLoop<=iSqrt; iLoop++)
if (iPrime % iLoop == 0)
{
args.IsValid = false;
return;
}

args.IsValid = true;
}

This is code I borrowed from another site to test this method - and it
works. My problem is that I need the JS to execute a VB function in
my project and I don't know how to do that. I want to do something
like:

function CheckValue(sender, args)
{
var sPartnumber = String(args.Value);
if FindExistingPN(sPartNumber)
{
args.IsValid = false;
return;
}
args.IsValid = true;
}

....where FindExistingPN is a funciton in my VB class. I have seen
some other posts about this, but none of them really gave me any
sample code that I could run. As I mentioned earlier, my JS skills
are lacking, so I am unable to create this myself.