OS News has an interesting article: The second operating system hiding in every mobile phone It discusses the security implications of the fact that all cell phones run two operating systems. One is the OS that you see and interact with: Android, iOS, Windows Phone, BlackBerry, etc. The other is the OS running on the baseband processor. It is responsible for everything to do with the radios in the phone, and is designed to handle all the real time processing requirements.
The baseband processor OS is generally proprietary, provided by the maker of the baseband chip, and generally not exposed to any scrutiny or review. It also contains a huge amount of historical cruft. For example, it responds to the old Hays AT command set. That was used with old modems to control dialing, answering the phone, and setting up the speed, and other parameters required to get the devices to handshake.
It turns out that if you can feed these commands to many baseband processors, you can tell them to automatically and silently answer the phone, allowing an attacker to listen in on you.
Unfortunately the security model of these things is ancient and badly broken. Cell towers are assumed to be secure, and any commands from them are trusted and executed. As we saw at Def Con in 2010, it is possible for attackers to spoof those towers.
The baseband processor, and its OS, is generally superior to the visible OS on the phone. That means that the visible OS can’t do much to secure the phone against these vulnerabilities.
There is not much you can do about this as an end user, but I thought you should know. :)