How it works
When someone makes a call, SIP messages help locate the recipient and negotiate the session. Once the call is established, voice packets travel over a separate media path.
Business example
A salesperson calls a customer from a softphone. SIP connects the application to the company PBX and sets up the call, while the speech travels through the media channel.
What to consider
Successful calls depend on registration, routing, NAT and network conditions. TLS can protect signalling and SRTP can protect media when both endpoints support them.