Simple Mobile uses the fourth largest Cell Phone Carrier in the USA for it's service. Simple plans cost a fraction of most Nation Wide providers. Simple $40 Unlimited offers; UNLIMITED Local, Long Distance, 411, Voicemail, Caller ID, Call Waiting, plus UNLIMITED U.S. & Global SMS. GSM sim card to start Simple Mobile Wireless service. Works with most T Mobile & Unlocked AT&T handsets. Simple Mobile uses the second largest GSM (coast to coast) carrier in the US.