With BBM 7, users are able to make calls to other BBM7 users who are on a WiFi network. This looks to change with BlackBerry 10. A screenshot has surfaced showing the settings for BBM in BlackBerry 10.
As you can see, there is an option for allowing BBM calls over the network connection. If you don’t have a reasonably sized data plan, this could get pricey. It will be interesting to see how carriers handle the new BBM call feature in BlackBerry 10.