You will find that your latency times will be unaccepable for voice traffic. Typically, the "magic number" you want to shoot for is roughly 125mS of latency or better. Although....cisco claims that the last space shuttle that went up used softphone via satellites and it worked fine....I wonder what fine really means?

Agree with the above comments that you'll get delay, but don't forget there's numerous queing techniques you can use to prioritise the voice traffic. Might be worth a try to see how much you can squeeze out of the circuit.