Here's the command I used to generate my new custom outgoing message for my iPhone's voicemail:
say -o outgoing-message.aiff -v Vicki ". . . Ahlec Jacobson \
did not answer. Please try emailing him, at alec jacobson at \
gee mail dot com. That's A. . L. . E. . C. . J. . A. . C. . O. . B. . \
S. . O. . N. . AT. GEE MAIL. DOT. COM. Or send a text message \
to this number. Or finally leave a message after the beep. \
Thank you. . . . . . . . . Good bye. . . ."
Here's the result after I used lame to convert the aiff file to mp3 using this simple command:
lame outgoing-message.aiff outgoing-message.mp3
Download mp3 I used this professional setup to record the mp3 onto my iPhone (after some cleanup with audacity and voluming boosting in vlc):