function checkForm(f) {
        var i;
        var phone = f.elements['phone'].value;
        if (phone == '') {
            alert('Please enter the phone number!');
            return false;
        }
        if (phone.length != 10) {
            alert('Please enter the 10-digit phone number!');
            return false;
        }
        for (i = 0; i < phone.length; i++) {
            if (phone.charAt(i) < '0' || phone.charAt(i) > '9') {
                alert('The phone number must contain only digits!');
                return false;
            }
        }
        return true;
    }