Removing sim cards does NOT remove your device from the cellular network. Phones without sim cards can still dial 911 at least here in Canada and I believe in the USA too. Thus they are still on the network, thus can still be found by their IMEI, and thus hot mic'd.

Here in Canada and I'm guessing in USA too 911 operators can push a phone calls into a special emergency mode so it doesn't get hung up. Even if the call to 911 was terminated.