Hi, 

I have tried as advised but did not work. A new entry created. Please check below code if I missed something.

------ Script Include (Client callable - true)------

var checkMobileno = Class.create();
checkMobileno.prototype = Object.extendsObject(AbstractAjaxProcessor, {

checkMobileno: function() {
var ans = 'false';
var mno = this.getParameter('sysparm_number');
var rec = new GlideRecord('u_visitor');
rec.addQuery('u_mobile_number', mno);
rec.query();
if(rec.next()) {
rec.u_state = 'checkedout';
rec.u_checkout_time = current.u_checkout_time;
rec.update();
ans = 'true';
}
else{
ans = 'false';
}
return ans;
},
type: 'checkMobileno'
});

-----Catalog Client Script (On Change - u_mobile_number) -----

function onChange(control, oldValue, newValue, isLoading) {
if (isLoading || newValue == '') {
return;
}

//Type appropriate comment here, and begin script below

var ga = new GlideAjax('checkMobileno');
ga.addParam('sysparm_name', 'checkMobileno');
ga.addParam('sysparm_number',g_form.getValue('u_mobile_number'));

ga.getXML(checkMobileDetails); 
}
function checkMobileDetails(response) {
var answer = response.responseXML.documentElement.getAttribute("answer");

var answers = answer;
if(answers == false)
{
alert("Please check your mobile number");
return false;
}
}

Thanks