I think your best bet is to try out two unlocked voip adapters (no need to emulate a phone line simulator). Set them up for direct IP dialing and enable modem pass-through mode. This will provide the least compression (essentially straight PCM audio) and lowest latency. It still might not be enough, but as far as I know, no one has tried it out.
I was going to try it out myself a few years ago, but VOIP boxes were still a little pricey for a project that might not pan out. Last I checked, they're dirt cheap on eBay but I don't have two Saturns anymore (still have two netlinks).
Good info on what modem pass-through mode does: http://www.cisco.com/c/en/us/td/docs/io ... _psthr.pdf