Wow, Mel-O-Tron! That's fantastic. I know that according to the rules I should come up with a whole 'nother interpretation but yours is so good I am totally convinced by it. And here's another instance of it: Ennis, at least after the divorce, didn't have a phone. (He had to send a card to announce his divorce, and then use a pay phone to call Lureen.) So yet more evidence that Jack's dreams don't have a chance with Ennis, so ...
OK, wait, wait -- another interpretation is suggesting itself. Or actually it's sort of the flip side of Mel's. Maybe phones, or lack thereof, represent Ennis' obstinacy toward Jack's offers. Not on your fucking life would Ennis change his mind. Ennis doesn't own a phone because he's not willing to listen to them. He does talk to his boss on the phone, but that's because he puts work as a higher priority than his relationship with Jack. But in the end, after talking to Lureen, he embraces the phone, realizing his mistake ...