Either a client or a server can initiate request in SIP. How is it possible? How will the server know about the client?
Since SIP entities have public addresses (e.g. firstname.lastname@example.org), a server may send an INVITE to the user's public server (example.org). The client will usually REGISTER a private address (IP and port) with its public server when it starts up, so the public server will usually forward the INVITE request to that address.