I agree with this. Mobile phones have defintely destroyed (some) peoples manners! Mobile phones are a useful tool, but some people are glued to it, and I do get quite irritated by a few people I know...
One of my adult DC has theirs with them all the time, and when they visit us, they can't go a full minute without 'checking' it, and having a little scroll though to see if they've got any new WhatsApp messages, or text messages, or emails etc. 2 hours they're with us, the first time they've visited in a month, and they have their eyes on their phone for some two thirds of the time they're here. I know it's an addiction, so I try to not get too annoyed, but it is a bit irksome. They still do manage to chat to us and listen to us and so on, but they have one eye on their phone, around 65% of the time they're here. Weirdly, they don't look at it much when we visit them, or if I meet them for a coffee... Just when they're at ours. Still a bit annoying though!
I also see a friend (ex colleague) for a coffee every 4-6 weeks, and she does pretty much the same. (She is in her 50s, unlike DC who are late 20s,) which surprises me, as most people who are Gen X are not addicted to their phone. She has her eye(s) on her phone about 50% of the hour and a half I am with her. Also answers calls, and texts, and WhatsApp messages while I'm with her.
Most of the time there is no need to respond straight away. It's her husband phoning to ask 'what time is tea?' or her daughter asking if she can get a lift to Shannon's house on Saturday (3 days later,) or her mum asking her to bring a load of bread round as she's nearly run out. They actually phone!
I find it really rude. She spends an hour and a half with me, maybe 8-9 times a year, and can't put her phone on silent/DND for that very short time. Also, I don't know about her mum, but her daughter, and her husband know she is with me, but still ring her. I don't know who is more rude, them for ringing when they know she is with me, or her for bloody answering! 