adayinthelifeof - from experience, phones aren't checked without wanting to find a reason why DH/DW/DP get very secretive suddenly or start EA or acting severely out of character.
FWIW - I never checked my stbxh's phone, he did suddenly become so attached to it though and spent hours on it secretly and when confronted said he was looking at the weather! I knew something was going on but he spat, "it's all in your fucking head".
I have never put passwords on my phone and I trusted my H completely until he started - seemingly out of the blue - to have had a violent, abusive personality transplant.
I discovered proof when I found a 5 year written diary that he kept of his plans with OW, he bought one for her too. I still didn't look at his phone but when I informed my MIL, she said I had no right to read his personal thoughts regardless of what my suspicions were as maybe he wasn't ready to tell me.
I hope I trust again but the phone is a dead giveaway as you can see from previous posts.
Most of the posters on this thread have only looked at the phone when things are so bad that they need to find an answer.
Would I check a future partner's phone? I don't think I would but I would like to think I would know if they are protective over it.
I think unless you've been there - you don't know how desperate we are for answers.