An IP phone connects like any other network device, it'll need an IP address, a Gateway, knowledge of what DNS servers to use, and find a time server (SNTP).
Extra things that an IP phone needs is to be able to find a provisioning server. This server could be an TFTP, FTP or HTTP(S) server, that provides configuration files, new images and log files for your IP phone to be able to boot up.
Once booted, the phone will also be looking for a registration server (wherever you have voice services configured). This could be a local "PBX" like an Asterisk server, or it could be a service like voip.ms
Certain models may vary depending on the manufacturer, but they basically all work the same way.