Will the new Ubuntu Phone support VoIP through the operating system when out of range of the network the user has? This would be a great way to steal market share from Apple which doesn't support VoIP except through complicated apps.
It appears as though the default VoIP client for Ubuntu/Ubuntu Phone OS is currently Empathy:
In addition to this, someone else was interested in something similar. Unfortunately, they found that VoIP is not a big feature with the current version of Ubuntu's phone os.
The VoIP client appears to work off of 3G/WiFi - type networks. You may have to start a new call after leaving the range of the cellular network. I don't think it's as automatic as you were hoping for, sadly.