I think Anne Hathaway's performance in the phone scene is nothing short of brilliant.
Despite having to wear all those ridiculous wigs, she portrays Lureen as hardened over time. There's the initial exasperation on answering the phone (before she even knows who it is), as if to say, "oh, not another one". We can imagine she's received many, many calls asking for information.
Thus, her very rehearsed explanation (whether or not it's true; it's the "family" or "public" version).
I love the squeak or break in her voice when Ennis corrects her about what Brokeback Mountain was. It's a transformation: I think this is when she realizes who Ennis really is ; simultaneously, she is realizing who she isn't and what she has lost or even never had with Jack.
Another squeak of recognition when Ennis says "we was good friends" seals it.
I forgive Lureen for her coldness. To paraphrase Annie, she learned some hard lessons about life by being with Jack. Plus, she's suddenly a widow and can't be much more than 39 herself. Now, whatever suspicions she may have had about Jack are confirmed, so she's discovering someone else was Jack's priority.