Leaving a stolen i-phone as security, no doubt.
I want to get paid, I don’t want a cell phone. What the heck am I going to do with a cell phone???
(1) If a con artist offers to leave a phone, frequently the mark will not actually take him up on it, either because it makes the scammer seem more trsutworthy or the mark thinks "what am I going to do with this phone, what a hassle" or both.
(2) Pretty much every legitimate phone comes with a replacement option if it's lost, so as long as you're scamming an amount of money that is larger than the replacement plan cost, you're still in the black even if you didn't get away with as much as you hoped.