[clivehawkins]

I'd go along completely with Mark on this one.

As it's a stand alone question there could be another answer, although I'm scratching my head as to the other three alternatives. I'm guessing (and only guessing) that one of them may be 'When I was walking into the office, the secretary was talking on the phone.' ie two contemporary actions. However, at this point I'd use WHILE instead of WHEN.

Anyone else?