The following documents (rightly or wrongly) how I added the Phone Number field to the Contact Us form.

I got the basic steps from a thread in the support archives, however they didn't work for 1.3.5 and required some additional customization.

The following will:
a) Add the Phone Number field to the contact us page
b) Fill in the phone number field if the user is logged in and has a phone number in their account
c) print the phone number in the email

These are the files I edited
./includes/languages/english/MYTEMPLATE/contact_us.php
./includes/languages/english/MYTEMPLATE/email_extras.php
./includes/modules/pages/contact_us/header_php.php
./includes/templates/MYTEMPLATE/templates/tpl_contact_us_default.php
./admin/includes/languages/english/email_extras.php

File 1: Insert the following lines
./includes/languages/english/MYTEMPLATE/contact_us.php
define('ENTRY_PHONE', 'Phone Number:');
define('ENTRY_EMAIL_PHONE_CHECK_ERROR','You must enter your phone number. Please try again.');
File 2: Insert the following line
./includes/languages/english/MYTEMPLATE/email_extras.php
define('OFFICE_PHONE','<strong>Phone:</strong>');
File 3:
./includes/modules/pages/contact_us/header_php.php
Add this to around line 15
$phone = zen_db_prepare_input($_POST['phonenumber']);
Modify the sql line around line 24
$sql = "SELECT customers_id, customers_firstname, customers_lastname, customers_password, customers_email_address, customers_default_address_id, customers_telephone
Insert this line around line 32
$customer_phone= $check_customer->fields['customers_telephone'];
...
} else {
...
$customer_phone='Not logged in';
Around line 50, modify the extra_info and text_message to look like
$extra_info = email_collect_extra_info($name, $email_address, $customer_name, $customer_email, $customer_phone);
// Prepare Text-only portion of message
$text_message = OFFICE_FROM . "\t" . $name . "\n" .
OFFICE_PHONE . "\t" . $phone . "\n" .
OFFICE_EMAIL . "\t" . $email_address . "\n\n" .
Around line 63, change html_msg to look like
$html_msg['CONTACT_US_OFFICE_FROM'] = OFFICE_FROM . ' ' . $name . '<br />' . OFFICE_PHONE . ' ' . $phone . '<br />' . OFFICE_EMAIL . '(' . $email_address . ')';
Around line 78, insert the code to test whether or not the phone number field has text in it
if (empty($phone)) {
$messageStack->add('contact', ENTRY_EMAIL_PHONE_CHECK_ERROR);
}
Around line 90, change the sql statement
$sql = "SELECT customers_id, customers_firstname, customers_lastname, customers_password, customers_email_address, customers_default_address_id, customers_telephone
A few lines down, insert
$phone= $check_customer->fields['customers_telephone'];
File 4: Insert the following in between the contactname and email-address field definitions
./includes/templates/MYTEMPLATE/templates/tpl_contact_us_default.php
<label class="inputLabel" for="phonenumber"><?php echo ENTRY_PHONE; ?></label>
<?php echo zen_draw_input_field('phonenumber', $phone, ' size="40" id="phonenumber"') . '<span class="alert">' . ENTRY_REQUIRED_SYMBOL . '</span>'; ?>
<br class="clearBoth" />
File 5: Insert the following define
./admin/includes/languages/english/email_extras.php
define('OFFICE_PHONE','Phone:');
Hope this helps someone (or me when I forget how I did it :-)