SIP is a peer-to-peer protocol where the roles client – server and exchangeable depending on who starts a session. In reality most deployments foresee a process called registration (method: REGISTER) which allows a central server (registrar) to store the location of a SIP User-Agent.
A SIP Phone is a client to the central Unified Communication Platform (registrar) – and the UC platform is a client to the SIP Server of the operator (registrar).
Once the SIP Server gets to know the location of an SIP Client, it can deliver calls and other messages to it from other Clients connected to the same Server.