Nokia X3-02 - Why do VoIP calls not succeed over the 3G connection in my Series 40 devices?

The VoIP connection over 3G needs to be enabled in Menu > Settings > Connectivity > Internet Telephone > Connection type by choosing WLAN or 3G. Also the VoIP settings need to have 3G support enabled which can be checked from Menu > Settings > Connectivity > Internet Telephone > Accounts > (your account) > Options > Details.