When requesting address settings from a DHCP server, each phone also requests additional information that the DHCP server may have by sending a Site Specific Option Number (SSON). If the DHCP server has information matching the SSON, that information is included in the DHCP response.

1600 and 9600 Series phones use 242 as their default SSON. However, through the phone's own menus the SSON it uses can be altered . For those phones using the IP Office system for DHCP, the SSON numbers that the IP Office supports are set in the IP Office system's configuration. The values used by the phones and supported by the IP Office system must match.