Physical collateral is in theory valid, but in practice it's total crap on this forum. There's gotta be a pawn shop in your area--it's frigging Dallas. You're not going to get as much as you're asking for it, but I highly doubt anyone here is going to accept a phone. Could be wrong, but I've been vulture-circling this section of bitcointalk for a while now and I rarely see physical collateral used.
That's going to be troublesome if someone wants to use a physical collateral to be used in a loan. Imagine sending the item which may take 2-3 days and waiting for confirmation, then once the lender or escrow received they will check the value of the item and decide if its acceptable or not. If borrower is in a hurry for Bitcoins then using a physical collateral is not a good idea.