I agree with the pp that if you have a trusting relationship there is no need to read each other’s messages. Though I think there is probably a difference between not doing it because you don’t need to and being banned from doing so, if that makes sense. As far as I know at least, dh has never read my messages and I’ve never read his - bearing like seeing them as they come through on his phone if I’m standing next to it. I’ve never opened his phone and gone through them. Because I’ve never had a reason to.
I think if you truly feel like you have a reason to, then you should and it’s possibly wrong but a necessary evil (I have done it with exes who I suspected were cheating and discovered in each case my suspicions were right). So my dh and I don’t have a policy where we have access to each other’s phones per se so we can check. But my phone isn’t password protected and I know his password (because he uses his phone for business, we run a family business, and I have on occasion needed to access it, it’s the same password he has used since about 2005, also his pin for his bank card, etc, so not a big secret).
So we don’t have a policy that we can check each other’s phones because neither of us feel we need to. We’re secure and have total trust in each other. But equally we aren’t prevented from doing it either. It’s just a non-issue.
That said, for anyone who suspects they are being cheated on, check away! No one deserves to be lied to and manipulated and when it’s happened to be in previous relationships I have. But that’s also why I’m not in those relationships anymore, because there was no trust. I’m glad I found out as soon as I could.