I need to add a new field to my custom form

not willing to use plugins.

Why?

Because your options are:

1) Use a plugin
2) Write the code in functions.php (which is exactly the same as using a plugin)
2b) Write your own plugin
3) Editing core (NEVER EVER DO THAT)

So … Why not a plugin?

I don’t understand what you’re looking to do here – why are you looking at the pluggable wp_mail() function? If you’re collecting a phone number, you need a form that a user enters it in, and then you need to put the phone number into the text of the $message that you send. wp_mail(), which is essentially just a wrapper for PHP mail(), doesn’t have a phone number argument.

@ipstenu dont want to use a plugin because all i want is just a single field (phone number), besides i dont want to create another page and change the design.
@helen i was just reading about you yesterday as i was doing a research about multiple check boxes.
1. apparently adding a new argument to the function wp_mail() is unnecessary, correct?
2. if so, how can i put the phone number into the text of the $message?

thank you guys!..

I don’t know where you are implementing this form, but this sounds like a basic PHP contact form. You build an HTML form, process the data in PHP, build a $message (which is just a string), and mail it. If you don’t know how to do that and do it securely, you really should reconsider using a plugin.

Thank you Helen i figured it out. yeah, you guys were right, adding another argument was crazy. So contact_number is the field that has the phone number.
<!——–>
wp_mail($email_to, sprintf( '[%s] ' . esc_html($_POST['contact_subject']), $et_site_name ), esc_html($_POST['contact_message']).esc_html($_POST['contact_number']), 'From: "'. esc_html($_POST['contact_name']) .'" <' . esc_html($_POST['contact_email']) . ' >');
<!————>
the field for my form looks like this:

<!------------->
<p class="clearfix">
<input type="text" name="contact_number" value="<?php if ( isset($_POST['contact_number']) ) echo esc_attr($_POST['contact_number']); else esc_attr_e('Phone Number','MyTheme'); ?>" id="contact_number" class="input" /></p>

<!————->
Thank you. Please look at it and let me know if you think i need to add something else.