He was writing an important report when the telephone rang,可

不能这样改啦
应该是：while He was writing an important report ,the telephone rang
while 的后面强调的是进行时,主句无所谓,但while 的后面一定得是进行时.
when 的后面则一定不能是进行时,主句无所谓.
while 和 when 就是这样的,他们是刚好相反的用法,记住一个,另一个当然也就记住了啦