I think that people could easily manage communicating with each other in the time spent outside of cars. We have always managed to find ways to communicate in the time before cell phones and we could do it again. I think banning cell phones and fining people who use them in cars would be a great way to get people to pay attention to the road.
Hands free technology has made talking on the cell phone while driving considerably safer than using a hand set. A hands free device is no different than talking to a passenger. So, while communication is possible without using cell phones, it would be severely limited without regard for whether the restriction is effective in making driving safer.