Could be your phone being pinged in order to keep tabs on your location. I would throw the phone away and change your identity. You must have upset someone in the government.

That or its nothing at all.

It's just pinging the nearest base station in response to a periodic poll of which devices are in the cell, like a 'stay alive' ping on a temporarily dormant FTP session. You can probably limit how diligently the phone keeps in touch with the network but will have more missed/dropped calls as a result.

Well, the only way to make 100% sure you're not spied on with your mobile phone is to turn it off AND remove the battery. I recommend this method for the paranoid moments in your life! ;-)