This would fall under the heading of evesdropping, which is frowned upon but not actually a crime. The more serious question is, what is this person doing with your phone in the first place? Theft of a phone is a crime. But maybe you just leave it lying around. Keep closer track of your phone.
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Definitely especially if the person you are aiming at is not your spouse or family. So don't do it.
Yes it is, we call it " Privacy Act" but sometimes if this person is our spouse, you can do it just don't get caught or you will get in trouble from your spouse..