Do you have any context? Perhaps it means that he's physically standing on a "wire" (like a tightrope walker). Figuratively, it can mean that he's hesitating and is unable to decide something (a much more common metaphor is "be on the fence about something"). That's all I can think of right now.

It may be an old-fashioned way of saying "he's on the phone."

That actually crossed my mind, but then again, we have no context.

Well, unless you're talking about a tightrope walker I think the phone interpretation is the most likely. I've never heard "on the wire" to mean "on the fence" ("undecided").

As tightrope/tightwire walkers walk on a rope/wire, they keep swaying side to side. And if they lose their balance, they are just as likely to fall to the right as they are likely to fall to the left. This conjures up an image of a person who cannot really decide which way to go or which option to choose, because both/all of them seem equally possible. However, I may be "all wet" on this one.

Without context - and if this is not meant literally, that is: someone is working or talking on some kind or electircal device, or someone is positioned on some kind of actual wire - I would guess this means the same thing as "on the ball," that is, sharp, smart, wise, etc.