People who know me know I am not one to tolerate rudeness. I am willing to play games by the rules established by the other party. My suggestion is to first consider whether the cell phone conversation is just annoying you because it is happening at all, or whether it is truly disrupting you. If it's just observably rude behavior, let it go with a brief praise to God you are not that troglodyte. If it is truly disrupting you, I suggest you respond proportionately and in kind. If there is a seat open at that table, or if you can pull a chair up, do so. Engage the person in the cell phone in vigorous, friendly conversation, at a volume that will disrupt their phone call but trying to keep it down so as not to disturb others. The longer and louder they talk, the closer you get to the person on the cellphone. When they hang up, thank them for doing so and return to your table.