Maybe she's someone like me. When my phone rings I want to answer it, because I figure someone is trying to reach me. I have a thing about people who don't answer their phones. I don't care who's trying to reach me. I just want to answer my phone.
If the thing is that you're friends with her husband, there's a chance she's so secure in knowing there's nothing between you, there's no reason not to answer when you call. If she's still a friend, there's no reason she wouldn't answer a friend's call either.