Both your dictionary and teacher are correct. As I'm sure you are aware, as in most languages there is more than one way to say the same thing, and also different ways of saying the same thing in different parts of the country. And lets not forget there is formal, casual and slang variants of many words. From what I can tell your teacher is trying to help you use the most commonly used way of saying things.

the best strategy is "trust no one" :)... the correct variants: computer = 电脑, doctor = 医生, mobile phone = 手机

I did the Open University Beginners Chinese course and in that course we were taught the term 电脑 for computer. That course has been written in the last ten years or so and I believe it to be up-to-date and reliable. We were also taught 手机 for mobile phone. Another teacher on italki has also taught me to use these terms. For doctor both my italki teacher and my OU course taught 医生 not 大夫, but I am guessing there are lots of terms for doctor as there are lots of types of doctors. The dictionary that was recommended on the OU course uses 计算机 for computer, nevertheless we were not taught to use this term during the course and 电脑 was used instead. Also if you look it up in any online dictionary or in google translate 电脑 is listed as a synonym for 计算机 and when any term like computer program, or computer game is listed the translation of computer becomes 电脑, for example computer game: 电脑游戏 and computer programmer: 电脑程序员 Hope this helps.

@brook: I see, thank you! o^_^o

大夫 means a doctor, especially 蒙古大夫 means a medical quack. We do not use it in spoken language, however, we understand it. 大 pronounces not ㄉㄚˋ but ㄉㄞˋ(the same as 代) here.