For me the issue is not the phone; the issue is how loud you are. If you can talk in a normal voice on the phone then I don't care where you do it in public.
Seems that people on cell phones in restaurants draw negative attention because they have to be loud to be heard over atmospheric noise. If you're loud enough to draw that much attention from all the tables that everyone in the surrounding area knows you're on the phone, then you must be pretty disruptive.
I just think it's rude to be that loud. People are spending money (perhaps saved up for a nice special lunch with someone or whatever) or they just want to relax on their break. It's rude to be disruptive. If you can talk without being loud & disruptive then I don't see why it's a problem.