This question asked is an interesting one and I'm very sure everyone in Nigeria can relate to this question. The question actually made me remember my last relationship but thank God I'm still alive. But before I answer all questions I will like to say a big thank you to @george-dee for letting me know there is a community on here that is created for my country.
To me checking your partner's phone is not trusting your partner, which is why I do I ask people why are you in a relationship with someone you can't trust. I know the drill, people will say they actually did one time and didn't do them well.
I laugh and tell them, that we all have a story, we all have a past, and throw this question to them why bring your past to your present, don't you know you are trying to ruin it?
Speaking of myself, I have never for once checked my partner's phone because I see no reason to do so, feeling too insecure will do nothing but ruin your relationship.
We have different chatting apps, and will you check all one after the other?
Each chatting app has its trick of privacy. If my partner sees her phone with me then she is about to get a debit alert.
If I notice my partner is keeping her phone away from me then I can become suspicious that something is going on, and it is truly an act of infidelity, she might not be cheating but she is hiding something from me or maybe she doesn't want me to debit her again.
There was this one time, I was hiding my phone from my partner just to avoid trouble, she don't want me texting any of my female friends even posting them on my WhatsApp status will cause serious trouble, and for that certain reason, I started keeping my phone from her reach whenever she is around.
I don't think there should be privacy in a relationship maybe your past actually, only if your partner's sense of humor is great and even if it's not I think you still have to tell him or her. Not keeping things from your partner will actually help you to build trust in your relationship but at the same time, it can be the cause of your breakup. Nevertheless, I think it is better to open up to your partner when you are in a relationship because if you are hiding things and he or she later finds out then what you are preventing will later happen and it might be worst than you ever imagined.
If there would be privacy it should be about your family matters, I don't think that needed to be shared with him or her or your friend's issues those should be private. You don't want to stain your family or friends in your partner's face.
I hope someone will come across this and definitely be able to learn something.