That’s right. The batteries should be dead. If the phones are underwater they wouldn’t be working, either. But this has happened to more than a dozen families; they place a call to a cell phone, someone picks up the call and then disconnects.
This occurred yesterday and was reported here on FR.
I’ve been following this situation for a while, it is now day three.
Sounds like an average morning at the NSA.
Just because you place a call and hear a ring doesn't mean the phone is actually ringing. Telcos will send ring tone to the caller just to assure them that they are still trying to connect. On successful calls, it's common for the callee to have heard fewer rings than the caller. This is because the telco played one or more rings to the caller before they managed to locate the callee's phone.
As for someone picking up and hanging up, that I hadn't heard. I'd expect at least an "All your PAX are belong to us" or such. What should happen if the callee's phone can't be located is to play a message to that effect and then transfer to voice mail.