header.php meta

I would like to know how to add a custom field to the header.php code that will append the title together with the contents of a field from a listing.

I have a custom field on each of my listings that contacts a phone number.
the field name is “tel_num”
the number is different on each page

I would like to add that into header.php
<title><?php wp_title(‘ : ‘, true, ‘right’); ?> <?php bloginfo(‘name’); ?></title>

so the page <title> tag is seen as follows

“The actual page title” : “contents of tel_num for that page”

Thus replacing bloginfo(“name”) for what ever I need to get the tel_num inserted.

Any help please.

Thanks