Two things should never, ever be done in a moving vehicle - texting and having sex. Neither will end well.
I have an aftermarket Jabra BT hands-free device in my car, that I will only use when it is safe to do so. I won't make calls, and usually only take them from close family members. To a certain extent, it is no different to talking with a passenger, but the big difference is that a passenger can see when traffic is busy (or weather is bad) and you are paying close attention and will (hopefully) stop talking. The guy on the other end of the phone doesn't see that.
However, like all aspects of driving, only experience (and to a lesser extent - age) can tell you when it is "safe to do so". Which is why many teenagers (the prime texting demographic) text and drive as they feel immortal and have been brought up to be competent multi-taskers in every scenario. If they can do homework, be on facebook, watch a movie and chat, all simultaneously, then why not text and drive a 2 ton machine at the same time. Those "LOL's won't write themselves.
Every part of driving a car is a risk assessment. You are constantly playing the odds. Is it safer to take that call or drive at 80mph? It all depends on the external scenario. Georgia has a phone law that is slanted towards the inexperienced, but is probably the thin edge of the wedge (No texting by anyone, and no phone usage by the under 18s, including hands-free)
The other thing many people fail to realize, in the car or in the house, is that just because the phone rings, you don't have to answer it!