The more Addie talks about her relationship with Lansing, the more curious Isaac gets about their whole dynamic. As a man who had married for love, Isaac thinks the bond between them seems so strange. Bringing a baby into that kind of environment is even odder -- because you gotta have sex to make a kid and there doesn't seem to be a whole lot of passion between them (that Isaac can detect at least from this conversation).
In his experience people only hid things when they felt they were doing something wrong but Isaac is willing to give Addie the benefit of the doubt. He's known her for a while, and though they've never been close, he likes to think that counts for something. That she might trust him enough to actually tell him the truth in all of this.
So she didn't know about Hawkins being the Dog King and she'd trashed the phone to stop her prying husband from asking too many questions. That just left... "Why don't you tell me about the volunteer work you've been doing down at the Department of Resources."