Wavelengths blocked by a certain metal mesh
It isn't the metal mesh leaking so much as the joint around the door. Rather than employing a continuous metal-to-metal contact around the edge of the door to seal it when closed, most microwave ovens use a quarter-wave "trap" &mdash; basically a shaped "groove" of the correct dimensions that surrounds the door opening (usually hidden behind a dielectric gasket of some sort) &mdash; to create a "virtual short circuit" for the microwave energy that prevents it from passing from the interior to the exterior of the oven.

This trap is fairly specific to the frequency that the microwave uses, and it isn't surprising that it wouldn't have much effect on cellphone signals.