Phoenicians are not allowed on the battlefield between the other three factions, however they are allowed to fight against factions that are not members of the Council of Venice. They also serve as protectors of civilians from the items listed in the Book of Secrets.
The Capitol City for the Phoenicians, New Carthage, they claim, floats in the sky.
Mentions of them are everywhere, especially once their quest line in Tokyo begins.
Kaoru in The Dream Palace is particularly forthcoming on the subject.
Tanis was a Council of Venice agent, but long before she is encountered in Egypt. She represents the Phoenicians now, as much to disrupt the plans of the "Red Team, Blue Team, Green Team", as out of loyalty.