Someone made a short HowTo document about loading SIP firmware into the cisco ip phones, models 7940 and 7960. He included SIPDefault.cnf and SIPmac_address.cnf for Free World Dialup, very close to what's needed for Voxalot. Although the original site is no longer online, you can see it
HERE at web.archive.org.
Here and
here are some Google searches that may be helpful.