On Sailfish X, ofono uses rild to control calls. Rild doesn’t support setting VoLTE. Sony’s provided rild uses propertary libqmi to control mobile network chip. There is also opensource QMI library implementation libqmi-glib ( libqmi ) that can be used to control mobile network chip. So QMI driver to Ofono maybe needs to be written. ( Voice enumerations and flags: libqmi-glib Reference Manual )
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I can imagine a couple more reasons:
- 2G has a much wider reach than 3G, 4G or 5G, which means you don’t need loads of 5G masts in remote areas to make sure people can place emergency calls
- you keep compatibility with pretty much all mobile phones, again making sure you don’t prevent (older) people from making emergency calls.
- (2G was so revolutionary in Belgium that we use the name for the technology, ‘GSM’, to refer to mobile phones. I can’t imagine a world where a ‘gsm’ will no longer be able to access GSM networks.)
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Yes, up to 25 km on land and up to more than 100 km on sea (in a special mode).
4G / 5G is in range more like NFC 
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@Maus following link says VoLTE would be working on Pinephone with Mobian:
https://wiki.mobian-project.org/doku.php?id=pinephone
Maybe it could be ported in any way!?
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Different hardware. So not likely. Pinephone also has the ability to run a FOSS stack on the modem, which Sony phones don’t (blobs). Keep in mind SFOS relies on halium/hybris.
Essentially SFOS uses Halium/hybris to translate into Android stuff. Pinephone can directly run mainline.
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Driver that supports VoLTE is needed to Qualcomm MSM chips that are use in Sony’s phones. And starts to look like that community needs to write it. Pinephone uses different manufacturers chips so that won’t fit.
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The Pinephone modem is a Qualcomm Gobi, i.e. a non-SOC modem packaged into an USB board by Qualcomm subsidiary Quectel. This choice is great from an isolation perspective, and is hell for developers at the same time. We should not talk about the Pinephone in this thread, because it does not help with bringing VoLTE to Sailfish in any way.
If I read correctly, what we need is a standardised rild that handles VoLTE – and if I got this right, this would be heavily device dependent (because implementation depends on the device’s modem). That’s a lot of work, I guess.
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Looked Android’s rild source to see how it works and what thinks there is for VoLTE. Rild in Xperia may already eaven have support VoLTE, but there is no code for using those features from ofono. Rild and it’s libraries are device specific. And it is controlled via unix socket standard android java code on Android and on Sailfish X it’s controlled by Ofono.
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So I’ve been experimenting with VoLTE workarounds for a bit and this weekend I fired up Emma and installed the stock ROM which I have done my best to degoogle and use without a Google account. .
Although carriers that are transitioning to VoLTE are delaying the shutoff dates for their 2g/3g services this doesn’t mean that they aren’t deprecating their existing networks to favor VoLTE/5G.
So certain issues are cropping up with calls going directly to voice mail and intermittently not being able to call out depending on your location.
As far as using an android VOIP app (text free) to manage voice and mms/sms over the data connection, it works when the Android data link is working.
However several times I’ve had this stop working. When you load Text free it will warn you of this and you can restart the android compatibility layer. But you won’t know it has stopped working until you check the app screen.
So it’s a combination of the lack of native VoLTE support and the instability of the Android app data link that has ended this experiment for me. In the end, what ever else it offers the fundamental talk and text ability of a phone must be unimpeachable.
I will miss the UI which is 1000x better than Android. Maybe someone will succeed with a native SIP solution or Jolla will release a VoLTE compatible release at which point I will give it another try. Otherwise it will be off to feature phone or pinephone land when this Xperia is at its end of life.
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Of course this setup can’t compete with a free Android app like Text Free on the price alone but it’s a Sailfish native solution and remember: If you are not paying for a product you are most likely the product.
There seams to be commits for ofono that adds some of P-CSCF (Proxy Call Session Control Function) features. So maybe VoLTE is coming is at least little bit closer.
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nexmo.com reroutes me to vonage…to a website that looks like a telecommunications system for a business?
" 1. COMMUNICATIONS APIS
- VOICE API
Voice API
Lead the conversation and deliver a better customer experience with the Vonage programmable Voice API. Personalize the WebRTC and PSTN calling experience with user context and data at your fingertips. Innovate, streamline, and scale with flexible voice capabilities—all on a reliable, high quality global carrier network billed on a per-second basis so you pay only for what you use."
I don’t mind paying for a service, but I have no idea how you could implement this as consumer on a personal cell phone.
TextFree is $4.99 a month for unlimited access.
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Well, effectively it’s just SIP and some web hooks for SMS. Maybe not something an average consumer would do but not rocket science either.
But it also probably depends on where you’re coming from - as someone who never owned any Android or iOS devices it was actually easier for me to set up than trying to install the TextFree app 
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I also was auto-forwarded to vonage and have the feeling this is more a service or offer to big companies and not intended for private people.
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I guess they have been acquired by Vonage at some point but the service has worked for me for years.
So I am making an app CellMon to see the signals.
What this tells me is that my 4G/lte is usually on 942MHz (sometimes in other low bands), and has a typical strength of 45% at home.
3G/umts is always on 2600MHz and strength ~= 10% at home. In particular, the fringe areas that now have no functioning coverage, are surrounded by bush. 2600MHz would be expected to be very heavily absorbed by bush, so this really explains why voice call range is now so poor.
VoLTE is needed not because there is no 3G, but because it is now unusable in many areas.
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Well, you can still run SIP over 4G without VoLTE, just saying 
But I have to admit, I’m a bit shocked with how many people seem to still be making/receiving actual voice calls. Is this still something that people under 68 would do these days? 
Uhm, yes. I am about halfway of the way to 68 and technologically well-educated, but I still make multiple voice calls per week. YMMV.
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Tens per day, a few per day on weekends… 
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