This sounds as if it were an insurance replacement rather than a warranty replacement. There is no "fee" for the warranty on your phone. Insurance thru Assurion, on the other hand has a monthly fee associated with it.
Unfortunately, Assurion does not guarantee the same phone you have when you get a replacement. If they have one in stock when making the replacement, yes, but that is not always the case. If you want the phone you returned replaced with an identical phone, you may be waiting for a long time.
I can't think of anything else you can do except to keep calling and checking to see if they have one of your model phones in stock.
Good luck.