To extend the iPhone battery usage time between charges, Apple decided to put the phone in a sleeping mode and detach it from the network, the instant it goes inactive. Now, if the user is not going to use the phone for a reasonable amount of time then, this is beneficial to both the network and the battery. On the other hand, if the user keeps “waking up” the phone every minute or so this can create a havoc on the network, since the phone keeps attaching and detaching from the network, which consumes a lot of signaling bandwidth to the tower, as well as draining the battery.

Also, in some cellular networks, some of the signaling from the tower is broadcasted to all phones in its range “Paging”, and only the intended phone acks back. I can imagine that there’s a lot of that in NYC, so your phone was getting a lot of “not-for-me” broadcasted signaling.

Finally, as for your 2Mb of “usage”, I suspect that it was signaling, but why was it clocking your usage meter?… In any case, I hope you wont be charged for it, as this sounds to me like charging a pilot for the airfare every time she flies an airplane!