This works on the premise that waiting with a reply is desirable outside of the context of texting, making the lack of it it notable within said context.
But this is not true – if I suggested that when someone approaches you in person to start a conversation, you ignore him a couple of hours before replying, you’d likely reject that outright – and if you didn’t, your conversation partner certainly would. The same applies to phone conversations – even through the ancient land line. Someone says something, you respond right away and don’t let the conversation drag on for hours with silence in between.

Responding to a social stimulus in a timely manner is not only taught to us from childhood (keep eye contact, respond to questions and greetings, send holiday cards and thank you notes), it is our nature. Social animals do the same. It is ‘social physics’: every action prompts a reaction.

I completely understand when someone puts aside his phone or turns it off, I am by no means saying that we have to be approachable/reachable every second. But if it is in the middle of a conversation, I at least expect a heads up – just as I expect one before my conversation partner hangs up the phone, or leaves the room.
Does that already cross the line to ‘addiction’?