Actually, there isn't a noticable loss of audio quality when it is recorded over the phone. When Al Gore recorded his lines for Futurama, he did it over the phone, I believe.
As the story goes, that's how Temuera Morrison recorded replacement Boba Fett dialogue in The Empire Strikes Back for the DVD. Supposedly some sound guys at Lucasfilm called him up, and asked him to repeat the words without telling what it was for, which is why it came off so flat.
