DHCP option 66 is useful for a VoIP phone to be automatically configured from a factory default state. DHCP will give the phone option 66, which points to the phone’s provisioning server. The phone can then contact the server to download its configuration or upgrade its firmware.
First, these options must be enabled from the Web GUI
Administration > System > Enable JFFS partition > Yes Administration > System > Format JFFS partition at next boot > No Administration > System > Enable SSH > Yes
Next, SSH into the router and open the jffs/configs directory
Create/edit dnsmasq.conf.add with vi text editor
Enter your DHCP Option 66 into dnsmasq.conf
Save that (ESC, :wq, ENTER), then reload dnsmasq