After the other tower went down:
10:33:42 ENTIRE TOWER DOWN The calls from the second tower ceased with another rumbling roar.
It appears that after each tower collapsed: 1) cell phones were unable to work in the vicinity, and/or 2) all the people were dead in each building. As very few people (I think the number I heard was five) were rescued alive from the buildings after they collapsed, I suspect it is a combination of both.