Respondendo minha própria pergunta:
Eu movi o arquivo /etc/asterisk/sip.conf criando um link simbólico. link
# 1. Delete /etc/asterisk/sip.conf we do not want that file on disk. It contains passwords!
rm /etc/asterisk/sip.conf
# 2. create sip.conf on memory (/dev/shm/sip.conf)
touch /dev/shm/sip.conf
... add configuration and passwords... to that file
# 3. Trick asterisk by placing a symbolic link.
# Point file /etc/asterisk/sip.conf ---> /dev/shm/sip.conf
ln -s /dev/shm/sip.conf /etc/asterisk/sip.conf
Não, quando eu acesso o /etc/asterisk/sip.conf na realidade, estou acessando /dev/shm/sip.conf!