I am not totally clear on how this works, but this is my understanding.  When you filed the original warranty claim and returned your original phone, they honored your claim and sent you a new phone.  At that point, the original phone no longer belongs to you.  It belongs to Verizon.  Normally, that is the end of the transaction. 

Evidently, when they received your old phone, they determined that it was not eligible for replacement under the terms of the warranty, so they gave you the option to pay $300 for your new phone.  The retail value is $550.  Rather than pay the $300, you decided to return the new phone.  Having done so, you are now even.  There is no reason they would or should return the old phone.  It no longer belongs to you, it belongs to Verizon.  Basically, you returned a $550 phone to keep from paying $300.  You gave up $250 which is more than the value of your original broken phone.