In the telephone analogy, a call must be initiated first by one party. The initiating party dials the number for the receiving party, and the receiving party answers.
This is also a common paradigm in networking called the client-server model. In this model, a server listens for connections. The client, knowing the address and port number that the server is listening on, establishes the connection by sending the first packet.
For example, the web server at example.com listens on port 80 (HTTP) and port 443 (HTTPS). A web browser (client) must establish the connection by sending the first packet to the web server address and port.
