Awww, does anyone else realize how sweet this is? This guy is doing a long-distance relationship and at least he's doing the right thing--trying to go on a nurse forum to ask for advice on how to be a better boyfriend. How many guys do that? You all are so blunt and hard on him. At least give him credit for making that effort!
Now, to be fair, there are other possibilities.
She's could be extremely ambitious and type A and for her, carrying on a long distance relationship with only a 20-40min phone call a week does it for her. I mean if it's like, pretty consistent, always on a Sunday or Saturday night especially on a weekend night I'd say it's maybe more possible she's saving the best of her weekend times to talk to you before burrowing back to work. Is she also working on the side? That might also factor in as that's exhausting, especially if she's doing a night shift it could throw off her schedule. What is the quality of the conversations, is it wooden and boring like she's just doing her duty and checking in, or does it feel like she's been penting up and ready to talk and let loose with you, and interested in your life? You don't have to answer, just think about it.
I only talked 1x a day to someone I really liked, sometimes for hours, sometimes just for a few minutes on a given night. But as long as there was a check-in I was happy and didn't need too much. I didn't like the phone and the thought of getting stuck for more than 10-15 min on a phone didn't appeal to me. Currently, I'm working full-time and taking 2 lab courses, and that's like enough to make me feel pretty overwhelmed. I'm talking with someone I met online but just exchanging an email 1-2x a week does it for me for now. As long as the communication is quality communication and shows the other person is as interested in talking to you, responds promptly, and is engaged, I'm happy for the time being.
But...if the problem is: she could be happy as a clam getting her Waki-fix once a week for 20-40 min, but Waki isn't happy with that. Then that's a legitimate thing to address anyway, since it's a mismatch of needs. One person may be more independent and the other may need more affection. Of course, you should discern the situation first, to see if it's just how she's carrying on happily in the relationship, or if she's (as others are suggesting) dissociating gradually.