Simple definition
A SIP URI identifies a communication endpoint, commonly using the sip:user@domain format.
Purpose and use
A SIP URI identifies a user or service, for example sip:phone@example.com, rather than only a local extension number.
How does it work?
Domain, parameters and DNS mechanisms help select destination and transport. Common ports are 5060 for SIP and 5061 for SIP over TLS, but configurations can differ.
Example and verification
For incorrect routing, compare the full URI, domain and rewriting rules applied by the PBX or provider.
Points to watch
The sips scheme concerns signaling protection; it does not prove that RTP media is encrypted. A displayed address is not proof of authenticated identity either.
In short
A URI identifies a logical target whose resolution, transport and authorization still require validation.