My son has used his phone responsibly since middle school - about 5 years now. (well, except for the time he had it out in school & got caught & had to pay the price/consequences for that. :dumbass
He is 17-1/2 and will be a Senior in HS next year. I don't check his phone for anything anymore. I did in MS and 1st year HS. Maybe I *should* but I seriously do not have the time for that kind of thing. I trust him now, but he has shown me over the last few years what his character & nature is and I don't have a reason to mistrust him. He's just a basically good kid with good morals, good character and behaves accordingly.
Of course, he is still a 17-1/2yo teenager, prone to doing stupid pet tricks.
He also knows that I will check if I feel the need to do so and he knows I am a stealth ninja about that sort of thing (checking on him when he least expects it!) so that kind of keeps him in check. The phone is paid for by dad, and we have the understanding that we can and will make it all go away if need be.