For mobile number validation you can use regular expression which give you best output . This is javascript code which check mobile number valid or not.

<html>
<head>
    <title>Mobile number validation using regex</title>
    <script type="text/javascript">
        function validate() {
            var mobile = document.getElementById("mobile").value;
            var pattern = /^[\s()+-]*([0-9][\s()+-]*){6,20}$/;
            if (pattern.test(mobile)) {
                alert("Your mobile number : "+mobile);
                return true;
            }
            alert("It is not valid mobile number");
            return false;

        }
    </script>

</head>
<body>
Enter Mobile No. :
<input type="text" name="mobile" id="mobile" />
<input type="submit" value="Check" onclick="validate();" />
</body>
</html>

and you can also jQuery for mobile number validation , this is simple code

<html>
<head>

    <script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.2.1.min.js"></script>
    <title>Mobile number validation using regex</title>
    <script type="text/javascript">
        function validate() {
            var mobile = $("#mobile").val();
            var pattern = /^[\s()+-]*([0-9][\s()+-]*){6,20}$/;
            if (pattern.test(mobile)) {
                alert("Your mobile number : " + mobile);
                return true;
            }
            alert("It is not valid mobile number");
            return false;

        }
    </script>

</head>
<body>
Enter Mobile No. :
<input type="text" name="mobile" id="mobile"/>
<input type="submit" value="Check" onclick="validate();"/>
</body>
</html>

If you face any problem then please inform me.