The telephone boots in the network and sends a DHCP request for an IP address.
DHCP server accepts the request, assigns an IP address and sends it back in a DHCP reply.
Besides the IP address, the message also contains TFTP (Trivial File Transfer Protocol) server address - PBX, or HTTPS URL (including authentication information)
SIP phone connects to TFTP/HTTPS server integrated in PBX
PBX server contains generated configuration file suitable for the particular phone
The phone is configured using the values it has acquired in the configuration file and after automatic reboot is ready to be used.