Now we're into the world of some other language. In your browser turn on the javascript console (in Mozilla you just type javascript: into the location bar not sure how to do it in IE). You need to find out what kind of error happens when you click on the Submit button. I suspect since your calling ValidateForm(this.form) when you click the button, your function should be modified to use the passed variable...

function ValidateForm(foo){
       alert("In JS");
        if ((foo.Mobile.value==null)||(foo.Mobile.value=="")){
                alert("Please Enter your Phone Number");
                foo.Mobile.focus();
                return false;
        }
}
[download]

Also in your code I supsect inFOrm should be inForm which might just fix your problem without rewritting the function like I just did.