Wikipedia has en overview of the SIP protocol
and the RTP protocol.
These two protocols are the foundation of SIP telephony. SIP is used to initiate connections between phones, and RTP transfers the voice data.
SIP can be transported on different underlying protocols, it is possible to use UDP, TCP or TLS for SIP sessions.
The different protocols have different pros and cons, but TLS usually works best in most networks.
TLS is an encrypted protocol that hide the session information.
Also the voice data over RTP can be transported with different underlying protocols, but in practice UDP is used
since low latency is more important than data transfer without errors. If a small number of data packets get lost
this will hardly be noticed.
It is also possible to encrypt RTP.