Jolt Mobile.
It’s an MVNO (mobile virtual network operator) that uses AT&T’s network.
Its service only works with an unlocked tri-band/quad-band
GSM 900/1800/1900 frequency phone.
The phone uses a SIM card, which Jolt Mobile sends to you.
If the phone’s service carrier is Cingular or AT&T,
then you may use a locked GSM phone.
If the phone’s service carrier is any other,
then you must use an unlocked GSM phone.