A recently leaked ROM for the BlackBerry 9900 revealed that RIM was likely going to be adding Wi-Fi calling for T-Mobile customers, but it was unclear what kind of Wi-Fi calling users would be getting. CIO reports that the 9900 will not be enabled for UMA calling — which allows for seamless handoffs between cell and Wi-Fi networks — but will rather be using a software-only solution built into the OS. RIM has supported the hardware-based UMA option in its product lineup for some time, and it's not clear if the change here is based on hardware limitations or simply an implementation decision by Waterloo and T-Mobile. The carrier, which offers software-based options for Android phones as well, stresses that there are battery savings with the approach, since the cellular radio is turned off when Wi-Fi calling is active. However, there's no denying it takes away an element of convenience if you have to stop and restart a call every time you leave your apartment. There's no timeframe yet on the release of the software update.