function p52_register_quote_meta_boxes( $meta_boxes ) {

    $prefix = 'p52_quote_';

    // Get all customer custom post type posts
    $customers = get_posts( array(
        'posts_per_page' => -1,
        'post_type' => 'customer'
    ) );

    // Define main meta box
    $meta_box = array(
        'id' => $prefix . 'customer',
        'title' => 'Customer Info',
        // Should define post types to display in array.
        // If you want this displays in Post, then assign to post here
        'pages' => array( 'post' ),
        'context' => 'normal',
        'priority' => 'low',

        'fields' => array(
            array(
                'name' => 'Name',
                'id' => $prefix . 'name',
                'type' => 'post',
                'post_type' => 'customer',
                'placeholder' => 'Select a Customer'
            ),
            // Real field
            array(
                'name'      => 'Phone',
                'id'        => $prefix . 'phone',
                'type'      => 'hidden'
            )
        )
    );

    // Append phone or another field of customer to meta box above.
    foreach ( $customers as $customer )
    {
        // Show this phone field if match name.
        $meta_box['fields'][] = array(
            'name'      => 'Phone',
            'id'        => $prefix . 'phone_' . $customer->ID,
            'type'      => 'text',
            'std'       => get_post_meta( $customer->ID, 'p52_quote_phone', true ),
            'visible'   => array( $prefix . 'name', $customer->ID )
        );
    }

    $meta_boxes[] = $meta_box;

    return $meta_boxes;
}

add_filter( 'rwmb_meta_boxes', 'p52_register_quote_meta_boxes' );

// Save data to phone field
add_action('rwmb_before_save_post', function($post_id)
{
    // Get person ID to save from "Select a Customer" field
    $person_to_save = intval( $_POST['p52_quote_name'] );

    // Save related field to phone field
    $_POST['p52_quote_phone'] = $_POST['p52_quote_phone_' . $person_to_save];

    // Unset all hidden fields
    foreach ( $_POST as $key => $value )
    {
        if ( strpos( $key, 'p52_quote_phone_' ) )
            unset( $_POST[$key] );
    }
} );