even though the feature was never enabled.. someone figured out how to unlock it! got to love it.. I recently looked at gettin a dash vs the blacjack which I sold and Im back on the E61.. but one of the reasons I quickly said no to the dash was for the lack of a SIP client even thogh it has wifi.... here are the instructions from howardforums
As expected, tmobile wm6 does not have voip enabled by default.. However with the help of ppl over at xda, I was able to get it working !! Here are steps
1) app unlock your wm6 (same surrealnetwork file works in wm6)
2) download voip cab files from xda here http://220.127.116.11/d900/patch/WM6VoIP.CAB and put it on your dash. then install it. You should see these files in windows folder
3) follow the steps here http://forum.xda-developers.com/showthread.php?t=299950
4) reboot your device and Settings->Phone->internet calling set the option "Whenever available"
5) it takes some time for the special sip icon to appear on your phone