The Dell Voice app on my iPhone 3GS works fine unless I actually raise the phone to my ear. It fails exactly when the proximity sensor is activated by this action (this is the sensor that causes the phone's screen to automatically dim when you hold it against your ear.) If I hold the phone away from my head so that the sensor is not activated, the call continues as normal. The speaker phone function also works, again because the proximity sensor is not being triggered.
I have tried uninstalling the app, then rebooting the phone, then re-downloading the app and installing it, but the problem persists.
A little more detail:
When I dial a number, I can hear the ringing start before I bring the phone up to my ear, but as soon as the sensor is triggered, the ringing stops at both ends of the call. When I take the phone away from my ear, the sensor turns the screen back on. I can see that the call is still in an "active" state and the time counter is still running, but nothing else happens. Just dead silence.