Missing well known actress in IMDB's cast list of movie "Public enemies" 2009 by Michael Mann

Sigourney Weaver can be seen from 1:44:19-28 & 1:45:27-33

On 1:45:26 she is asked "Miss Roberts, can you reach Agent Purvis?" which, according to someone in IMDB’s GOOFS section declares that the name "Miss Roberts" is wrong… WITHOUT TELLING WHAT THAT NAME SHOULD BE!!! What's the use of telling a mistake if you don't even give the answer?!?? Furthermore the name Sigourney Weaver is mentioned nowhere in the CAST LIST.

Guess that's enough trouble for today isn't it?

My question: 

Who can tell me what role Sigourney Weaver plays and, if not Ms Roberts, what her name should be; as well as, optionally, how can such an important actresses be forgotten to be mentioned in IMDB’s Cast List which is incredibly long and full of totally unknown actors?