Please describe your networking environment (modem make/model, router make/model, etc.) Most modems can issue an IP address via DHCP and most routers include a DHCP function.
If you have a computer with two NICs, you can set it up to e.g. share the Wi-Fi Internet connection to the Ethernet port. DHCP is enabled by default. You may need a crossover cable or switch in the path, if the Ethernet NIC is not auto MDI/MDX.
Some free programs, such as TFTPD32 for Windows, include a DHCP server as an auxiliary function.
If the intended application for the OBi will use the Phone port, it's simpler to just get a phone that can send DTMF.