You should have to wrote the java script function to do the phone number validation
<input type="text" name="billing[telephone]" value="<?php echo $this->escapeHtml($this->getAddress()->getTelephone()) ?>" title="<?php echo $this->__('Telephone') ?>" class="input-text <?php echo $this->helper('customer/address')->getAttributeValidationClass('telephone') ?> validate-mobileno" id="billing:telephone" placeholder="Mobile"/>
<script type="text/javascript">
//<{0,1}(\-){0,1}(\s){0,1}[1-9]{1}[0-9]{7}$";
// var matchArray = v.match(timePat);
if(v.length > 0){
if(v.length !=12){
return false;
}else if(v[0] != 9 || v[1] != 1 ){
return false;
}else if(v[2]!=9 && v[2]!=8 && v[2]!=7){
return false;
}
return true;
}else {
return false;
}
}
]])};
var dataForm = new VarienForm('form-validate', true);
//]]>
</script>