As an explanation of why people like to use international call-back sim cards, here's a good article that appeared in the International Herald Tribune a couple of years ago that describes how Riing/United Mobile operates:
http://www.iht.com/articles/2005/06/...ess/liecht.php
Phones/Wireless Devices: Doogee S90, Isatphone Pro, Amazon Kindle 3G, SkyRoam MiFi device, Karma MiFi device, AT&T Liberate MiFi device
Sim Cards: T-Mobile (Mint), AT&T (Mifi device or Kindle), Koko
Satphone: InMarSat
Broadband US Wireless Data: AT&T postpaid, Sprint (Karma Mobility prepaid)
Broadband International Data: SkyRoam
VOIP: Skype