The SIP server is the CPU IP address for VoIP connectivity. The phone is attempting to register to the CPU.
Press FNC + 6 on any phone and note the VoIP address.
At the IP phone, press HOLD + TRF + * + #, arrow down to the password section, dial 6633222 (nec) for the password.
Select SIP settings, Server address, 1st server address and type in the address found above. (EX: 172.016.000.010) Save, exit, the phone will reboot. If your system is set up for IP phones with a VoIP daughterboard, it will register.
NOTE: At least 2 static IP addresses are required. One goes into MB 10-12-09, and the other goes into MB 84-26-01.