You OWN your home, even though its mortgaged. You are not leasing it. You are not renting it. It is yours to do with ANYTHING you please. The bank has a lien on your home if your are mortgaged. They do not own it, nor can they tell you in any way what to do with it. You can sell it for any price you wish or burn it down, but before the government will grant the deed to someone else, they must verify the lien has been removed.
Likewise, when I purchase a phone + contract, I own the phone immediately. The phone company counts it as an immediate sell on their books and all other accounting, which is why you pay taxes for it immediately rather than over time. The sell of the phone is complete at that time in every legal way that matters.
Its not a lease, its a sale. If it was a lease, you would be required to buy it at the end of your contract ... that is a LEGAL requirement for leases, they can't automatically be 'given' at the end. The purchase price can be one dollar (which you'll find on all sorts of deals, Lease at XXX amount, buyout for $1, great corp accounting cheat ;), but a new transaction must take place for the lease to become a sell. They aren't doing that, again making it clear the sale happens at the start of the contract.
If I were renting or leasing, the price of my monthly bill would go down when I stopped leasing or renting, like if I bought a unlocked, no contract phone. But it doesn't. My bill remains the same regardless of where my phone comes from. Again, it can't be a lease or rental if there is no transaction and length of terms.
They have structured it as a sell in every way for their accounting purposes, but they want to pretend its not, again for their own financial interests (keeping you tied in to their service).
As far as 'defaulting', they are not the legal framework designed to deal with that situation. They are not supposed to be enforcers either. They have early termination charges attached as an agreed on termination cost. Not allowing your phone to not work on another provider ISN'T IN THE CONTRACT AT ALL, so its not something you agreed on ever.
If I pay $600 cash in $100 bills to Joe Smith who happens to own an unlocked iPhone on the street corner, the phone company treats me the same in EVERY SINGLE WAY as the guy who gets a contracted iPhone. So you get treated like you bought it ... again, they treat it as a sell for their benefit.
Only when it may benefit you is it suddenly not a sale to you but some other rental term bullshit they made up.