I don't think it boils down to whether they should touch each other's phone or not. It depends on the bedrock through which a marriage is built. It's the wrong questions because sometimes we need to consider the behavior and tolerance of two people in a marriage I believe when two couples understands their goals, boundaries and limitation, things like phone privacy is what they'll come to an understanding of their free will or limitations.
Also, there are many kinds of couples and I believe privacy between couples should be premeditated, that is from when they're dating and yet unmarried. They should have come to a consensus as to whether they should try and share everything between themselves or not and when this definitely happens they may agree or disagree so as not to have misunderstandings in fact privacy doesn't really guarantee fidelity or infidelity in a marriage that is why I do believe before people go into marriages they should defend their goals and stay true to them because before you could actually have access to a person's phone then they definitely deserve permission whether you are their spouse or not, so like I said in the beginning sometimes it boils down to understanding.
Personally, I feel that your spouse should have access to your phone at all times of the day. When you're a couple, your business is no longer just your business. Your business, your friends, every aspect of your life is shared with someone else so it's only right for you to share access to your phone which is basically the electronic hub of all your affairs with your partner.
Of course, just like all other things in life, there might be situations where you might want to hide something from your spouse, like a suprise or information that they're not yet ready to know. These are things that happen. But for the better part I see no reason why your partner shouldn't have access to your phone. The only reason you'd want to hide phone from your spouse is to hide things from them. And if their finding out would jeopardize your relationship then you shouldn't be doing it to start with and you're already wrong.