Cliffs: your phone sends radio signals sometimes, and when the tower tells the phone that you've got a call, the phone sends radio signals back so you can take that call.
Ever notice how if you hold a microphone real close to a tv/monitor, you can hear a buzzing sound through it? Electromagnetic (EM) fields.
Same thing. Electromagnetic fields are electrical fields and magnetic fields. A way long time ago (relatively) some famous guy figured that electicity creates magnetism, and magnetism can create electricity. This is how we get our power.
Anywho, to save power, your phone is not always sending out radio signals, unless you're on a call or dl'ing something (rings/pix/etc) with it. It sends signals to the tower, the same as irc does (Ping? Pong!), and then, when it activates its radio transmitter, is when it causes interference. Same as when you call or get called, the tower's radio transmitter sends out for just your phone, and since your phone is always passively listening, it gets it right away. Then it makes contact, a response that it got the message, callerid info, and that it will ring. It stays connected in case you want to take the call.
Notes: Passive listening (think sonar) is easier, uses less power, and more logical for the mobile device, than checking in every so often. Yes the device checks in once in a while, but if it had to check in to get the call, it would take about 5min for someone calling to get to your phone.