A phallus is an erect penis.
A symbol is a representation of something, by convention or resemblance: the digit "2" is a symbol for a quantity.
A phallic symbol is a (more or less abstract) representation of an erect penis.
"Ancient phallic symbol of masculine virility" is a phrase in Angels and Demons, the first of the "Robert Langdon" cycle by author Dan Brown.
It is a tautology: "ancient phallic symbol" would have been sufficient to express Mr. Brown's idea. Demonstration: "phallic symbol of feminine virility" or "phallic symbol of masculine femininity" would be self-contradictory to most readers.
Mr. Dan Brown's brave fight against elementary rules of style has been rewarded with 80 million sales in 40 languages (2009 data)