Worse, GSM encryption is pretty broken, people could be listening in, etc. Phoning people is still sensible in that attackers have to be close to the target, though (or get a wiretap).
Basically, as long as you have enough signal such that the victims handset can hear your commands, you can tell it that you are stronger then any other signal, and the handset will instantly switch to your cell.
What's worse is that there exist secure GSM encryption/etc. standards, however as the cell tower chooses what encryption to use, it is mostly pointless. (I remember watching a video about this. Apparently your phone is meant to show a massive warning if they are using unencrypted GSM. None do).
http://www.youtube.com/watch?v=rXVHPNhsOzo is a good video about this.