Cell phone manners are horrible many times. Went to eat at a restaurant and across from me sat a woman on her phone and four other people in her group. She chatted loudly the whole time I was there. I learned about her dog, what she ate that morning, when her next doctor appointment was, how the dining room painting was coming along, and how little Bud is doing in his first year of high school. If I had been one of the others in her group, I would have excused myself and left. If you don't have time for your other guests, I would consider myself not that important to you.
It is also annoying to other people sitting around you.