When you do a transaction, the controller tries to queue up a set of transactions from its other ATMs. It will either succeed or timeout. In either case, the transactions are communicated to a 37X5 and from there to a mainframe which runs a batch job to do the transaction. Its just that the whole transaction is usually sent to the mainframe, and that is slow going. Save the ATM receipt, as they normally ask for it.
Diabold ATMs check the deposit envelope to see if there is anything in it.
What's happened is a mass move into cellular by criminals. I have a few comments to make on this and some related topics which may be of interest to Net readers. My comments are based upon personal knowledge and experience as one who has provided some forensic science consulting services to certain law enforcement agencies for a number of years. It's sort of interesting to note that it was even easier to implement spoofing fraud in dial IMTS mobile telephone installations, but such fraud has been virtually unheard of. Progress in telecommunications has unquestionably been of benefit to criminal activity.