I no longer feel obligated to listen to or speak to anyone with their cell phone out. If they have an important message, they can excuse themselves to take the message or to send one and then put the phone away. But what happens is they believe they have a right to control the flow of conversation; when they have something to say, you are supposed to listen, but if you try to respond they go right back to their calls and messages and then swing back to you when they feel like it. They have zero obligation toward you, their companion.
The only way IMHO to reduce this abominable behavior, this lousy cell phone etiquette, is to ignore people when they have their phone out. That may include conveying important information to them like, “I’m not taking you home just now” or “I changed my mind about paying for your food/ticket/whatever.” Just remain silent until they put their phone away. Even that is rude as hell because they are still saying, “You just shut up until I need to speak to you.” But at least some communication may occur.
The Emily (or is that e-mily?) Post rules are OK but they avoid the obvious control over the conversation so many users try to impose. Screw them.
This is not different from household or workplace etiquette: someone is always trying to yell rather than to locate the person and talk to them directly. If someone is yelling something at you because they are too self-centered to get off their ass and come to you, you are under no obligation to respond (unless it’s the boss).