I think you should bring it up.
It might help to remember that this is a common problem people have in long-distance relationships, whether they're poly or not! I was in a long-distance relationship with my now-husband for a few years, and it was pretty brutal, and he didn't call me frequently enough for my tastes, either.
Don't let him trap you with "he's always liked how "laid back" I am... I don't want to appear any less laid back." It's dangerous to be complimented on how low-maintenance you are. They like you because you don't make any demands and you don't make a scene! That kind of thinking can prevent you from bringing up real issues when they come up. Don't worry about losing your "chill chick" status if you can honestly bring up a few problems, with calmness and understanding. You'll still be a chill chick.
Okay, so, next time you're on the phone, you're going to have to ask him if you can talk about something. When he says yes, tell him that he calls you quite a lot less frequently than he used to, and that you are wondering what it means. Tell him that you are fine with him cycling through girls and having other relationships, but that you'd like to stay on his list and that you're afraid of losing him. Just like that, as straightforward and calm as you can (easier said than done, I know).
And then see what he says. Don't push for some sort of solution right away, because you just sprung this all on him. It can just be something to think about and talk about tomorrow, or in a week.