I found this excellent article explaining how cellphones work.
http://db.tidbits.com/article/9796
One very interesting thing that popped up is why sometimes you’ll get a VoiceMail notification even when the phone didn’t ring (and also why texting will work when you can’t call). Basically, text and notifications are sent on a dedicated channel, so even when the regular voice channels are all busy, you can still text and receive notifications.
Early on when they invented GSM, someone decided it would be useful to dedicate a small part of this signaling to sending messages to your phone. They added a feature to send 160-character messages over the signaling channel. The initial idea was to use it to alert you to new voicemail messages, but then someone thought it might be nice to also send some short text messages, and thus the Short Messaging Service (SMS) was born.
That’s why you sometimes get voice mail notifications without hearing your phone ring. If the local voice channels are all filled, the call can’t get through and callers are forced to leave a message, but since the notification uses that signaling channel, it still reaches you right away. A nice side benefit is that SMS messages will often go through even when regular calls won’t. When I’m wearing my part-time hat as a disaster worker, I often find myself using SMS when I can’t make regular calls. If you are at that big concert, game, or Steve Jobs keynote you might try SMS instead of battling your neighbors for scarce voice channels.