It sounds to me like you're speakers or speaker wires are picking up your GSM synchronisation / send receive signals from the nearest cellular tower. You could try just Googleing (is that the correct word?) "shielded pc speakers" to see what you come up with.
But the best suggestion I can offer you is simple to move your phone to a different spot on your desk. If your speakers are in between your phone and the cellular tower, then you will pick up the interference every few minutes. Try a few different spots to sit it if you can - for example, above the level of the speakers is a good spot.
Specially shielded speakers and cabling will probably just cost you a lot more money.
Hope this helped!!
Cheers,
Rob.