Research In Motion (RIM) has gone over the top with BlackBerry Messenger and Voice calling with the addition of BBM to BBM user calling according to FierceMobileContent.
Basically, this means that calls between BBM users are free, anywhere, and part of the data plan. Imagine if Apple did that or Google/Android did this, the carriers would be livid. But this is RIM and they have been a true partner with the carriers for years, sharing revenue, using them as a channel for distribution and more.
This is a very good thing and with BB10 a few months away in an era of declining voice minutes the traditional way, what RIM is doing is one way for the carriers to still see some of the revenue.