For those who want my final code
http://somesite.com/page/?agent=1
$post_id = false;
if ( isset( $_GET[ 'agent' ] ) ) {
$post_id = intval( $_GET[ 'agent' ] );
$value = get_field( "phone", $post_id );
if ( $value ) {
echo $value;
} else {
echo 'empty';
}
}
}