Most Helpful Guy
It's not a good thing for someone to be present with another and occupy themselves with a phone. It's rude actually. The quick check of the phone is okay, a quick text sure. But the phone goes away for 90% of the time. If he does this along with not responding to your texts. It's showing a sign of disinterest. Whether this is a conscious decision he's making or he's not aware of how it's affecting you is anyone's guess. It's still disinterest and I'd bring it up. There's also the consideration of how often do you text? Because everyone needs time alone and to be in social contact with your girlfriend for the majority of everyday of every week is straining for anyone.
Talk to him about the phone while you're together. And if you hardly ever text him and he's not responding, then talk about that too.1
Most Helpful Girl
Seems a bit shady to me but who knows? He could be talking to a side hoe just as well as he could be talking to a friend. It still sucks though that he doesn't really pay attention to you while you're together. I'd mention it to him and see what he says1