gzt: H÷ is part of the 3G standard. You're on 3G for those calls.
I had a thought about the lack of VoLTE. I presume if you're in an area with those towers that are on 4G only, then you still have data. I have a 2Talk home phone line and I was able to load a SIP client on my phone to access that number. So when I am in an area where I can't make voice calls because there is no 3G I can always make a call from my 2Talk account. I would not leave the app running all the time as it would mean all home phone calls would appear on my phone! So people might not be able to call me when I am in that area but can always leave a message which I can pick up later.