How can I validate a single textbox in different different tabs

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ASP.NET

Inside a form I am using a textbox as "Mobile no" and i want to validate that textbox in different different tabs because i am calling the same form in all the tabs:
the tabs are:
1.Prepaid tab
2.Postpaid tab
3.Dth tab
4.data card tab

now when user navigate to first tab,second tab or fourth tab the mobile no textbox should accept only 10 digit but when fourth tab is called it should accept any no of digit

What I have tried:

function IsNumeric(e) {
var specialKeys = new Array();
specialKeys.push(8); //Backspace
var keyCode = e.which ? e.which : e.keyCode
var ret = ((keyCode >= 48 && keyCode <= 57) || specialKeys.indexOf(keyCode) != -1);
document.getElementById("error").style.display = ret ? "none" : "inline";
return ret;
}
function ValidateRecharge(rdvalue) {
debugger;
var MobileNo = document.getElementById("txtMobileNo").value;
var amt = document.getElementById("txtAmount").value;
var errorMsg = '';
if(errorMsg==''){
if (MobileNo == "") {
errorMsg += "Please Enter Mobile No.|";
}
//else {
// if (MobileNo.length < 10 || MobileNo.length > 10) {
// errorMsg += "Mobile No. is not valid, Please Enter 10 Digit Mobile No.|";
// }
//}
if (amt == "") {
errorMsg += "Please Enter Amount.|";
}
}

if (errorMsg == '') {
document.getElementById('btnRechargeNow_Click').click();
}
else {
document.getElementById('lblErrorMsg').innerHTML = ShowMessage(errorMsg);
return false;
}
}