thats pretty much it...
much like your internet provider, the phone company can only handle a certain ammount of data at one time, so much like your internet provider, when alot of data is flowing through it, yours slows down...
Same reason as if you try to phone people at midnight on new years eve, you often can't get through, because so meany people are having the same idea. Its simply a question of capacity. or something along those lines...
what HANDzCLEANx said really...