The way is useful in addition to that, and often IME far more important than the what. IMO you should document both, as necessary, and that's the hard part/skill. You have to treat it like you won't remember it in 6 months, because in 6 months, you won't. A lot of devs don't do that.

(A fair number of devs won't follow the style guide / idiomatic patterns of a language if there isn't a CI check forcing them to, and will argue to all ends with a human as to why it ought not apply to them, to the great detriment of their code's readability to other experience practitioners of the language. Oddly CI enforcement usually works better, why I don't know.)

So then the question that needs answering is:

"why" is the what not obvious?

Document the "why."

What is the phone number for? When I read it, is it ready for display as-is or should I be formatting it? Same question when I set it. Can it be null or empty? Does or can the system use this number for anything - such as sending SMS messages?

Being static is a huge red flag, though, and even though it's private I'd still ask. Why is it static? Is this thread-safe (and how)? What context does it get set in, and when can I use it?