Hope these links aren't a repeat. Here are some links I have found. If you google "Shannan Gilbert911 call there are many more.
Several reliable sites describe Shannan's 911 call, some saying the police told the mother the details of the call,
one saying "Police say Gilbert even specifically named her attacker who was someone she knew."
See
http://www.nj.com/hudson/index.ssf/2010/12/missing_jersey_city_woman_iden.html
This includes a video interview of the mother and the sister. They do not say they heard the call directly.
Then there is
http://www.my9tv.com/dpp/my9_news/suspected-long-island-serial-killer-'met-victims-on-craigslist'
"We all heard the last 911 call," said Gilbert's mother, Mari Gilbert. "[Shannan] was screaming the person's name [and saying], 'He is trying to kill me!' She was saying, 'Help me! Help me! Help me!' She was trying to escape. We know who it is."
I had many interruptions trying to get this posted. Sorry it is rather sloppy.