Vincent- This sounds really weird and interesting. Iíll get you started with an educated guess, but to get a real expert you might look for some electrical engineer.
Assuming that your description is accurate, obviously you are right that thereís some interference. CRT computer monitors (is that the type you have?) are notoriously sensitive to electromagnetic fields, since the electrons on the way from the filament to the screen feel electromagnetic forces.
It is not plausible that the E-M fields are those from some central towers, or satellites, etc., since they would then affect lots of screens, not just the one of the person getting a call. So the fields must be coming from your phone. Probably it sends some strong response when it picks up a signal intended for it, identifying its presence and readiness to receive calls.
Itís interesting that the interference goes away during the call itself. That means that the E-M signal from your phone is much weaker during the call. I suspect (but you should check with someone who actually knows) that the phone ínegotiatesí a signal level with the tower, so that once the connection is made the phone sends signals just strong enough to be easily picked up. That might help a lot in avoiding unnecessary power use and hence battery drain. On the off-chance that the EM radiation might be bad for you, it would also be good to avoid unnecessarily intense levels.
Of course there may be some more boring explanation. Maybe the low-frequency signals to the phoneís ringer/vibrator are radiating some. That would also stop after you answer. You could check this possibility by seeing if the timing of the patterns on the screen matches that of the rings.
The phone may have to negotiate with more than one tower, if it is in an overlap region (or even if it isnít). Itís probably the towers and associated electronics and switching facilities that actually handle the decision of which tower gets to handle the call, but they need a good, clear signal which can be separated from the background to help make this decision. Also, the phone and the tower donít initially know how far apart they are or how good the radio contactís going to be, before the call starts. If the phone is far away from a tower (but still can pick up it signals because the tower puts out a very strong signal or is amplified by some fluke of the geometry of buildings or local topography), the phone has to send its reply signal extra strong to make sure the call doesnít get dropped even before it starts. As Mike notes, the signal level can be stepped down once the proper level is determined. You could go the other way, increasing the power from low levels until the signal gets clear, but that would add extra time at the beginning of the connection, and cell phone users are notoriously impatient.
(published on 10/22/2007)