What you were told is wrong.
at&t puts an iPhone owner under contract only when the iPhone purchased from Apple or from at&t is activated with at&t - which must be done by the buyer thru iTunes.
I am under contract with at&t - signing a new two year contract on my iPhone's activation date. My iPhone was exchanged under warranty during the first month. I did not sign a new 2-yr contract when activating the exchange. If my iPhone was lost or stolen and I purchased another, I would not be required to sign a new 2-yr contract for my replacement. The contract is not tied to a particular iPhone since the iPhone is not subsidized by at&t in the traditional way. If my iPhone was lost or stolen, I could purchase a cheap go-phone as s replacement and cancel the unlimited data plan for my iPhone with no penalty, as long as I fulfill my contract which doesn't have to be with an iPhone.
The folks at this at&t store who told you this baloney must have been sleeping during this part of their training. Hard to believe they remain so clueless about this after nearly a year.
I also have a business phone with Verizon so I have constant side by side comparison. "Can you hear me know?" - is as much if not more of a joke than "more bars in more places". I left Verizon for personal use due to crummy reception - they let me out of my contract a year early because of this at my house, and sine of the places I frequent most.