Ask the Experts; May 2008; Scientific American Magazine; by David Grier, Susan Trumbore; 1 Page(s)

It sounds like a case of electromagnetic interference, or EMI: radio waves emitted by one device causing undesirable behavior in another. Virtually every piece of electrically powered equipment acts as a radio transmitter, whether it is supposed to or not; the changing electric currents running through these devices naturally radiate electromagnetic waves. This radiation is an inevitable by-product of harnessing electricity to do useful things, analogous to the clanking and clattering of traditional mechanical devices. Computers are particularly "noisy" because they rely on rapidly changing currents to act as clock signals that coordinate their calculations.

One possible explanation is that your computer unintentionally emits radio waves in the range of frequencies reserved for cell phone communications, typically around 800 megahertz (millions of cycles per second). If the signal coming from your computer were strong enough, your phone could mistake it for a cell phone transmission--albeit an indecipherable one.