TLS has the potential to protect communications between parties in a SIP call. I believe it is one of the standards that will grow in the near future, mainly because of the adoption of NGNs (next generation networks) in the Telco companies. It is possible to protect the communication from users and to VoIP providers. I recommend reading about this. TLS does not encrypt RTP packets, but it can be used to exchange keys for SRTP (secure RTP). SRTP can be used to encrypt SIP traffic.