How to validate mobile number in javascript

How to validate mobile number must be numeric and 10 digits.

<script type="text/javascript">

function ValidateMobNumber(txtMobId) {

 var fld = document.getElementById(txtMobId);

 if (fld.value == "") {

 alert("You didn't enter a phone number.");

 fld.value = "";

 fld.focus();

 return false;

 }

 else if (isNaN(fld.value)) {

 alert("The phone number contains illegal characters.");

 fld.value = "";

 fld.focus();

 return false;

 }

 else if (!(fld.value.length == 10)) {

 alert("The phone number is the wrong length. \nPlease enter 10 digit mobile no.");

 fld.value = "";

 fld.focus();

 return false;

 }

}

</script>

<input type="text" id="txtMB" onblur="return ValidateMobNumber('txtMB')" />

 

<!--Or Call on Button-->

<input type="submit" id="btnVal" value="Save" onclick="return ValidateMobNumber('txtMB') />