Wrong: they ALL apply.
If you receive merchandise that you didnt order, you have a legal right to keep it as a free gift.
You have no legal obligation to notify the seller. However, it is a good idea to write a letter to the company stating that you didnt order the item and, therefore, you have a legal right to keep it for free.
The question is what should you do, not what you are legally required to do. The first two statements still apply: 1) you have a legal right to keep the item and 2) you have no legal obligation to even write them a letter. In fact, the second answer also recommends to write a letter - telling them you are exercising your right to keep it.
OP: you got gypped, son! Exchanged an iPhone for a bluetooth headset you might not even use, or a model you might not have even bought.