Latency.... (Score 2, Informative)

ITU-T recommendations for toll-quality voice are 150ms round-trip latency which you've got Buckley's chance of getting across a Sat link. That being said, if your expectations are that you are using a Satellite phone, then much higher figures might be quite acceptable. I second the vote for Skype. The iLBC codec it uses degrades very gracefully over low-quality links.