At first please open functions/theme-mail.php and under:
$email = htmlspecialchars( stripslashes( trim( $_POST['Email'] ) ) );
please add below code:
$telephone = htmlspecialchars( stripslashes( trim( $_POST['telephone'] ) ) );
and replace:
$subject .= ', from: ' .$name ;
please paste:
$subject .= ', from: ' .$name. ', telephone: '.$telephone ;
after that please go to template-contact.php and under:
<input id="Email" class="email required" name="Email" placeholder="<?php _e('Your e-mail','doover'); ?>" type="text" />
please add below code:
<input id="Phone" name="Phone" placeholder="<?php _e('Your telephone','doover'); ?>" type="text" />