mig33 has 6 million mobile users

[qi:033] mig33, a combination mobile instant messenger, chat rooms and international calling card service based in Burlingame, CA., says signed up six million cell phone users in 200 countries. The service is particularly popular in countries like Bangladesh where international long distance rates are high. Unlike other VoIP services, mig33 works on even the most basic phones, and don’t require a Wi-Fi connection for the calls.

mig33 was one of the eTel/GigaOM LaunchPad companies, and since then has raised $10 million in venture backing from Accel Partners and RedPoint Ventures. It has relocated from Australia to the San Francisco . Other mobile callback/calling type services that offer lower cost minutes include Jajah and Cellity.