The in-universe description is this:

By hooking up the nodes of the microwave emitter to the receivers for a cellphone, the Microwave-O-Phone manages to become a miniature particle collider, generating micro-Kerr Black Holes (which evaporate almost instantly). The 42" CRT lifter is the source of the sparking phenomenon, as when the door is opened, it feeds electrons into the black hole(s), causing their event horizon to disappear, and allowing data to be sent to the past.