I guess people have different reasons for eating out. I don't often go to restaurants, so for me its about spending time with the person/people I am with. So in this instance, I would think it rude if someone was texting at the table. Unless it was an emergency.
If the person with me would rather spend time with the person at the other end of the phone, then they don't need to be in my company at that time.
I always find it sad when people go out for dinner and then don't talk to each other. But I respect that other people have different values.
My children know not to text, talk, email whilst sat at my dinner table. We don't get much time as it is.