And I probably can assume that the Phone entity has other properties (instance members) as well besides the id property, otherwise it would be a very weird entity
I'm not a JPA (Hibernate) guru at all, but I've never done it as you have described. And I wonder why the other properties of Phone (Object B) are not changed to null if an existing primary key is used.
Could you share (a part of) the mapping of both entities? You can always use dummy class and property names.