MO-Call have launched a BlackBerry app which offers cheap international calling and SMS rates. The app seems to work much like a calling card, in that when you launch the app, it will re-route calls through a different number and use credits that you purchase at their site. The rates seem pretty competitive, and it lists the following for the US:
Fixed: $0.020/min
Mobile: $0.020/min
SMS: $0.124/msg
You can find out more about their rates from their site.
To download MO-Call, point your browser to mo-call.mobi or visit their site for more information.
