They certainly do have enough power as I can hear my phone in the speakers of my car radio, as well as the speakers on my computer, whether or not it's ringing.
Never heard any phone transmitt on any car speakers, sure you don't have a car fitted with electronics to interact with a phone, I hav one and that can happen even when not connected. Just the same it doesn't take as much for IMF frequencies to bleed into a car radio, especially the AM reciever. To alter computer memory you need EMF, no frequency, just pure magnetic power. Now their is some EMF energy involved with all radio tranmissions, especially a 700 watt microwave focused onto one small area.