GSM has one striking advantage, which is that you can activate any GSM phone
yourself (if it's "unlocked" or if it's "locked" to Cingular rather than
another carrier) by just inserting your SIM card into it. A SIM card is a
tiny "smart card" used in your GSM phone.

You can buy a used Cingular-compatible (unlocked or Cingular-locked) GSM
phone from anyone and make it yours by putting your SIM card into it. Then
it has your number and your dialing directory.