The one thing I hate about hangouts and I could never figure out how to change was when searching for someone to text. It would search googles list of people with google+ and not my contact list first. Random people I don't even know named Mark, for example, would show up for me to text.
This is the most infuriating thing...now I just go through my contacts to find who I want to message (since their number exists but selecting SMS still brings up "On Hangouts")
Still makes no sense to me.
I'm on iOS.