So some years ago, someone pointed out to me that Tracfone billing is done on the actual phone itself; not the network. So with that in mind, I gave something a try, had some decent luck with it, and figured I'd pass it on. At least on phones using AT&T's UMTS network (though I assume this applies to the CDMA phones too), general call forwarding is blocked as it should be, but call forward unavailable/busy/no answer has to be active for voicemail to properly work. So sure enough, using standard GSM call forward codes, you can send those calls elsewhere, and it won't deduct any minutes on the account. At this point, you can ditch the phone in a suitably shady manner, like sliding it under a vending machine at the airport.
One caveat with this is that the AT&T mobile network's toll trunks _suck_ (you may have better luck with some of the other carriers. I'd try Verizon if it's convenient/CDMA turns out to work). Compared to just straight 1+, these trunks are ridden with discomfort noise and latency. If you're willing to deal with this, at the very least, it will not be transcoded like normal cell calls are. I don't know for sure, but both of these may very well be avoidable anyway if you choose to forward to a number local to, or otherwise within range of direct trunks to the mobile switching center you're assigned a number on. Keep that in mind when you give Tracfone your zip code.