I assume that by attached pages you mean the related pages. Added e.g. to a page type field "Pages". In this case the values are not stored within the page object in the DB but in the binding table CMS_Relationship (very old design). This means that you need to request the cms.relationship object type and get the values for the left or right node of the relationship. So, the REST API call could look like this:
/rest/cms.relationship?where=LeftNodeID=16