According to Page 28 in following link-
"IF 'DHCP' IS USED IN THE IP SETTINGS, THE
DNS SETTINGS MENU WILL BE DISABLED."
But not for TFTP.
So I guess you must be getting the DNS info through DHCP but not with TFTP.
TFTP must be static I guess.
I have another Tandberg in another site that works fine with just DHCP addressing. It seems to have got the TFTP address from DHCP as well.
I don't know much about the Tandberg endpoints, especially from configuraion point.
So can't really say what the issue might be. Earlier what I found, I thoungt I would share with you.
Why don't you take help of Tandberg's Support Group. Just shoot a mail to them and they have pretty good service.