There are other threads where I've posted more detailed stuff on cell phones.

Eugene and EMPNotImplyNuclear are both correct that this is mandated by the government (not like you couldn't find that out yourself... figured I'd throw my knowledge 2 cents).

Susan, you can get "pings" but not be able to complete a voice call. This is why SMS messages can get through. At a cell site there is a control channel, and a number of voice channels. Your phone is constantly trying to listen to the control channel, and sends information to the tower on a periodic basis. If you send/receive an SMS message this is the channel that does that. Your phone WANTS to be connected to this channel ( ET phone home! with a ping! ).

Side note: You'll notice I said "always on". It is segmented into "buckets", and when you send a text message, it takes up a bucket(s). Those buckets cost you from $0.10 to $0.25. Whether you sent the message or not, the "bucket" is always there.