The widely used VoIP signalling protocol SIP has a bad reputation. In my opinion it's mostly unwarranted. If you don't stray too far off piste and have a decent NAT traversal behaviour things will generally work fine.
In the ideal world everything would be encrypted, but there's a plethora of end of life and unsupported devices out there, and devices which may support encryption are often configured by end users that may not enable it.
Enter Opportunistic SRTP - a method of encrypting the audio stream if it's supported by the other end, and it's not supported just fall back to plain old RTP