Introduction: I've heard about a theory, according to which you can check if there is any leakage in your microwave oven by putting your mobile phone inside and call it. However, I gave it a try and the call was successful.
I realized that the two devices are working on different frequencies (and wavelengths, of course), thus the metal mesh in the door of the oven may be unable to block anything different from the intended working frequency. I came across similar results on the Internet.
Calculating the free-space wavelengths for 1.8 GHz and 2.45 GHz, we get ~0.167 and ~0.122 meters, respectively. This means a 40% increase in wavelength for the unintended usage.
Problem: Still, I don't understand why I was able to call my phone. First of all, I vaguely seem to remember a 'thumb of rule', namely that a metal mesh blocks electromagnetic waves of which the wavelength is in the same order of magnitude as the dimensions of the holes (or smaller). Well, that looks absolute stupidity, as the holes appear to be no more than 2-3 mm each side (almost a hundredth of the wavelengths).
Secondly, those wavelengths are practically the same in comparison with the holes.
Question: Given the dimensions of a metal mesh, how can you determine the range of wavelengths blocked by it?