Cell phones are not at all like land lines. Land lines did one thing. let you call, be called and take voice mail. Cell phones and especially now, smart phones are personal information troves. They have information from where you've been, what you do in your daily life, contact information, etc. They are basically mini personal computers now and there is a good reason to lock them in general.

I personally find that there isn't anything appalling or questionable about wanting your private smart phone kept private from everyone. In today's world where everything is really easily accessible and open, there is a need for our own private space. With everything becoming digital, it even, for many people could hold their own personal diary which, I believe, no one here would have a problem with others being locked out of. Furthermore the smartphone itself is almost a diary of our own habits and not everything even between loving partners necessarily needs to be shared. even in a good marriage or partnership, some right to privacy should be held. The best place for one to keep such private thoughts and/or information would be a smart phone and personal computer of which, I actually do lock everyone out of.

I do not believe that in and of itself locking your personal devices, including phones from your partner is in any way a sign of someone being deceitful. note I did not say "hiding something" because frankly those words are misleading. of course if you keep our phone private you're hiding everything in there but that does not imply deceit or that that information is in any way wrong. Except in the case where a partner has proven to do things that were questionable (flirting beyond acceptable limits, cheating, or otherwise lying and deceiving) In that case, there may be reason to believe that in the spirit of building trust again that that person should keep their information unlocked. Under normal circumstances though I don't think it's wrong at all.

But again, this information that I share is subjective and based on my own perspective and belief about such things and honestly these things depend on the level of trust and the expectations agreed upon between partners and likely varies a lot between couples. In other words, the discussion should really be taken up with the partner to come to a compromise or agreement and no one here can really dictate whether the act of locking a device is acceptable or not for other people's situations.