I don't have Rogers so I'm not entirely sure, but it's quite possible you may have been using Rogers' network for data (especially if wifi is turned off). It's best to check with them and make sure.
By default I believe the device will use a wifi connection of the mobile one. I'm assuming you're using Opera Mini 5. There should be an option for wifi (checkbox) under Opera's network settings.
In order to make wifi calls, you need to be connected to a wifi hotspot. Go to your mobile network options>connection preference and be sure to select wifi only or wifi preferred.