The doctor asked us to call him during the night except in case of necessity.

The doctor asked us to call him during the night except in case of necessity.
医生吩咐我们,除非必须,否则不要在夜里叫他
为什么我觉得应该用ask sb not to do,这么翻译才对,难道上面翻译的错了?
这个翻译是在书上抄的,应该不会错.
到底是固定句型,还是怎么回事.这里的“否则不要”是怎么翻译来的