TLS for SIP and RTP has long been on my hit list. I've been traveling a lot more for work recently, so secure mobile VoIP has gone up in priority.
Step 1, create an SSL certificate.
openssl req -out certreq.pem -new -nodes -keyout key.pem
(Optional) Step 1a, sign the SSL cert with your own CA. Otherwise, get it signed by a trusted CA.