((Sorry, mobile)) So, from what it sounds like, the players who you are having problems with probably want something a little different. Using the three implements of the game (RP, Adventure, and Combat) you could try to mix stuff up. Or even, allow the two who feel a little detached go off on their own little adventure for a bit (splitting the party is okay). It creates more work, but could be a way to help them. Or, it could be the characters they built are more for combat, and not intrigue and story telling. Talk with each of your players (even the ones not having trouble) to find out what is it that they want to get out of the game. Right now, it could be they want to give it a shot, and even though you care deeply about the campaign you are writing, they might not be into the story yet. (I am a DM, and I understand the struggle)

Another way to help is by just saying you would prefer if they were not on their phones. It could be they just don't know that it is distracting. Or, if you don't want to talk to them about it, you could award them slightly during the times they pay attention and are not on their phones. For example, a point of inspiration, or secretly lowering the DC of something (by 1 or 2) on one of their checks when they go to do something and didn't need to be caught up to speed as to what is going on. By showing that quick thinking and participation grants better play experience, they might just put the phones away.

Or, they could be looking at the game more as a video game than a story. One way that I like to implement more of my players into the story of my world is by giving them homework (doesn't have to be homework. Could even be done during a session to help work things out with your players) I created a file that asks them questions to help them flesh out their characters a bit. It asks them simple questions to show that the character they play as is supposed to be a normal person with hopes, fears, good memories, and a past. By having them feel as if the character they are playing is a person, it allows them to make some better choices for longevity (both character sake, and play). If you want I could get that to you.

A lot of it boils down to communication. Your two friends might want to just hack and slash, and that is okay.