The grandstream information on option 66 is very terse. I played around with for hours until I found a working solution by trial and error.
I could not get tftp to work but did get http to work. Option 66 for the default provisioning port 83 on My FreePBX install is this below. This is for my Cisco DHCP server specifically: option 66 ascii "http://192.168.32.3:83/"
Putting 192.168.32.3:83/ manually in the phone provisioning field also works. Must select http for the provisioning AND the ending / seemed to be important in my case as well.