There's TLS for SIP and SRTP for encrypted RTP audio. Good fuarking luck finding anyone who implements it.
It still leaks tonnes of metadata and like any IP traffic is traceable. You would need to roll your own Asterisk box with TLS and SRTP, isolate it from the outside world and then access it using a VPN. Tor is outta the question; SIP fuarrrking hates any more than 100ms of latency and it eats soykaf and dies if that latency spikes up und down (jitter).
sauce: is a person wot earns CapitalismCredits by installing big enterprise VOIP systems.
also; nothing you say on a mobile phone or landline telephone is secure fuarking EVER.
at some point I may actually build for lains the above mentioned asterisk box with tls and srtp and openvpn for a laff. but then like any other supposed 'secure' service, you'd be taking it on good faith I'm not dumping all your rtp to disk (server has to have the keys in srtp/tls) and handing it over to %shadygubbermintspooks%