The point is not that the booth is "accessible to thepublic" at other times, but that it is a temporarily private place whose momentary occupants'expectations of freedom from intrusion are recognized as reasonable
It is important to note this sentence that is outlined because privacy is now implied. The question in this case is complicated by the definition of when the phone booth is a public versus private space. When the space is private it ensures that a person's privacy is protected under the Fourth Amendment.