In casual use, they are often interchangeable. In careful use, "when" is usually associated with a static description or a completed action, and "while" with an ongoing action. However, there is considerable variation in usage. WHEN you phoned me, I was working in the garden. I was working in the garden WHEN you phoned me. WHILE I was working in the garden, you phoned me. You phoned me WHILE I was working in the garden. When I was in France, I visited the Louvre. While I was traveling in France, I visited the Louvre.

Both are grammatically correct. The meaning is only very slightly different: "You phoned WHILE I was working in the garden." You phoned during the period of time that I spent working in the garden. (I was working in the garden from 1:00 to 3:00 pm. You phoned sometime during those two hours.) "You phoned WHEN I was working in the garden." At the time that you phoned, I was working in the garden. (At 2:13 PM, the specific time that you phoned me, I was working in the garden.)

While and when I almost don't have any difference in this sense. The only difference in the grammar sense is that we cannot use simple past after "while", but it is ok to use simple past after "when"