Wordpress - How to get custom post meta using REST API

WP API has a rest_prepare_post filter (or rest_prepare_CPT if you are working with custom posts) which you can use to modify the JSON response. In your case it will be rest_prepare_joblisting.

function filter_joblisting_json( $data, $post, $context ) {
$phone = get_post_meta( $post->ID, '_phone', true );

if( $phone ) {
    $data->data['phone'] = $phone;
}

return $data;
}
add_filter( 'rest_prepare_joblisting', 'filter_joblisting_json', 10, 3 );

Using the same filter you can also remove fields/data from the response and do any manipulation of the data.

$post in the callback function is an array, not an object. So you cannot use $post->id. Change it to $post['id'] and it should work:

function slug_get_phone_number($post, $field_name, $request)
{
    return get_post_meta($post['id'], '_phone', true);
}

I recommend to change _phone to phone_number or something else without underscore prefix. Because _ is often used with private meta keys. Try to add a custom field which has meta key with _ prefix directly to your post, you will see what I meant.

Just Add this methods to function.php

add_action( 'rest_api_init', 'create_api_posts_meta_field' );

function create_api_posts_meta_field() {

 // register_rest_field ( 'name-of-post-type', 'name-of-field-to-return', array-of-callbacks-and-schema() )
 register_rest_field( 'tour', 'metaval', array(
 'get_callback' => 'get_post_meta_for_api',
 'schema' => null,
 )
 );
}

function get_post_meta_for_api( $object ) {
 //get the id of the post object array
 $post_id = $object['id'];

 //return the post meta
 return get_post_meta( $post_id );
}