data monitoring - "Android OS" is continuously downloading something in the background, how can I stop that?

My HTC One V (on 4.0 ICS) seems to be continuously downloading something even when it's inactive, and it's using the battery abnormally fast.

My data usage graph:

^{(click to enlarge)}

On the first picture, the brown line is for the "Android OS" element, which as you can see is responsible for at least 60% of the data consumption. I don't know what Android is downloading, but it does it continuously and it has a big effect on my battery's life. When I disable mobile data, the battery usage line becomes flat (see second picture). How can I check what exactly "Android OS" is doing? And how can I disable this activity?

Here is the battery usage graph. As you can see, the line became flat when I've disabled mobile data.

^{(click to enlarge)}

Answer

I've found the culprit! I went to Settings > Call > Internet call settings and deleted the SIP account I had once created and forgot. The phone kept reaching the SIP server (probably to check for incoming calls).

I had to install a packet sniffer on the phone (tPacketCapture) to see where the requests where going and realize I had configured a SIP account.