When requesting address settings from a DHCP server, each phone also requests additional information that the DHCP server may have. It does this by sending a Site Specific Option Number (SSON) request. If the DHCP server has information matching the requested SSON, that information is included in the DHCP response.
By default, most Avaya SIP telephones use the SSON 242 to request additional information. Depending on the particular phone model, it may be possible to change the SSON number it uses.