I know this is a long shot as fongo or Dell are both not avid application developers but some key features that would greatly improve the user experience.
Integration with stock dialer. Some sip clients have this, what it means is you can dial from stock dialer and when you hit dial you are given option to make the call using dell voice or stock phone/wireless network.
Rotate to horizontal view. When phone is in horizontal position the app should switch to horizontal view allowing easy use if phone is dock in horizontal position.
Of course bluetooth working properly. Being unable to connect to my bluetooth stereo makes this application useless in my car as I am not incline to break the law and risk my own and the safety of others on the road just so I can take a call.
If all of these functions were properly implemented I could seriously see this application completely replacing my voice plan with Rogers and just keep the 6gigs of data. I could potentially save $45/month on my cell phone bill.
Please Fongo/Dell I would gladly pay $100 to have this functionality working due to the potential savings.