Security Features
TLS/SSL
See SSL/TLS specific guide.
SIP Security
Security related features in PJSIP, among other things:
SIPS
TLS transport
Various authentication: digest, AKA, AKA-v2, OAuth 2.0
See SIP Capabilities for details.
Media Security
See SRTP — Secure RTP (SDES and DTLS-SRTP) specific guide.