I think the prof and Ethel are overstating the case here
The basic idea is sound but innacurate and unreliable
The shielding in a microwave should act as a Faraday cage and stop the signal reaching your phone.
But mobile phones do not use quite the same frequency as microwaves and the intensity of the radiation generated by the oven is much higher.
Further this is only the oven space and would tell you nothing about the shielding where the microwaves are generated.
So I can see where the idea came from but I wouldn't rely on it
Somebody's talking about it here:
http://isgadget.com/how-to-know-if-your-microw ave-has-leaks-cell-phone-test-and-wifi-test/