Scott and I answer each other's cell phones all the time, or check each other text messages if one of us isn't in the room. For example, he just got a text and I didn't think anything of it to check his phone and see who it was from/if it was important before taking his phone upstairs to him.
One of my friends can't believe we look at each other's phones and said it was such a violation of person space and showed that we don't trust each other.
We completely trust each other, and just treat our phones as 3 separate lines (landline too) people can use to communicate with us.