Only IP addresses in the Toshiba IP phones. No DNS, domain names.
You can set 2 IP addresses in a phone, the primary and the secondary. What I have done is set the internal IP address to the primary, and the secondary IP address as the public WAN. When it fails on the primary, it will try the secondary IP address.

As mentioned, you can also just use the public WAN IP address for internal IP phones in most cases.