There won’t be any operator left in a few years but I guess a data-only SIM coud maybe work.
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From what is written in that link, VoLTE is done by the modem and “nothing complicated is required on the postmarketOS side.” The only job is to configure the Profiles.
And I cant imagine, that the “Xperia”-Sailfish phones are equipped with modems, that can’t handle that.
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So how can you use it let’s say on Xperia XA2?
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False. 3G/UMTS is dual-mode, just like 2G/GSM.
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True.
Whereas 3G can handle data and telephony at the same time. And 2G can only handle one at a time (stopping data transfer when calls are made).
So all of them are different in some aspects.
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Also false, but data over 2G is already not very much use.
Combining CS and PS puts even further limitations of the rates you can achieve (and isn’t always supported) but technically, it works.
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Indeed each operator has a list what type/model/branded/not-branded is supported. The beloved J1 hardware seems not ready for it.
Technically GSMA/3GGP standards apply, e.g. ‘IR.92 - IMS Profile for Voice and SMS’, where SIP is just one of a few “misc.” parts 
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ORLY?
Then there is a lot of fud on the net spreading this.
Maybe related to the circumstance that it ‘feels’ like there is no data at all when calling and using data at the same time?
I tried a quick search on the net but found no reliable source that would confirm or negate this…
Let’s not diverge this topic further. (Probably my bad for biting)
Feel free to use private messages, or ping me in some new thread if you wish to continue.
But this is that anyway:
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Thank you very much for this link and right keywords.
If you know what to search the internet is quite useful
Found this interesting read and it seems network needed to support it as well as the device itself. Nowadays it should be there.
But was it in the times? The wiki article states support for VF UK came only in 2010…
That may be the cause for this often found false statement.
I was not argueing with or against you. I just did not know better. 
And I am pretty sure we wiil not derail this topic while waiting for Jolla to implement VoLTE. But bring some more knowledge!
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VoLTE/VoWiFi can be implemented in LineageOS as in PostmarketOS but, to my knowledge, is provider-specific. However, maybe it is possible to borrow the required backend from the android implementation? However, a working, integrated SIP backend (not an app) would still be mandatory. I agree that the requirement for VoWiFi/VoLTE is getting urgent with the recent announcements, even though GSM will probably be available for another couple of years (at least in Europe).
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Do we know if the implementations in other OS:es actually involve a SIP stack running on the application processor? Wouldn’t those be subject to certification? Having profiles behind AT commands sure looks like the modem is doing much of the heavy lifting. I honestly don’t know.
However; that also means different modems come with different support levels and possibly even firmware differences between instances of the same phone/modem. I know Jolla distributes firmware for Jolla C and Aquafish, but for the Sony devices…? I think we are stuck with whatever we start with. With Android however, Sony could push whatever firmware updates needed together with userland integration.
Could be a serious quagmire trying to unify this…
I also don’t know how important these profile things are, but given that they exist, surely not insignificant. I can only imagine the outrage that would fill this forum if you were limited to one of 14 operators, most of them not in the EU, for (initial) VoLTE support.
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We should monitor how the situation in Russia develops, should telcos in RU decide to switch to VoLTE it’s almost guaranteed VoLTE will also come to SFOS
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No volte will probably force me to stop using sfos, as my cell provider seems to have fewer and weaker 3G signals, so voice coverage is becoming unusable in many areas.
They have not announced anything, but areas of good voice, suddenly become not-spots and stay that way. The 3G (voice) is 0-1 bar when 4G is 4 bars.
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I think maybe data-only SIMs may still work after T-Mobile and AT&T cease allowing non-VoLTE capable phones to access their networks. Surely not something everyone may want to do but ist would be a way for me and a few others to continue using SFOS.
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My issue is not that they won’t allow the phone, it is that the 3G service is degrading badly.
I think they are reducing the number of sites with 3G or having 4G only sites.
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While they are not exactly the same these issues may still be related. I guess the whole reason why they are thinking about banning non-Volte phones is to be able to get rid of all this expensive legacy 2G, 3G infrastructure.
VoIP calls over 4G run just fine in Sailfish so I guess I will just continue using SIP.
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There’s no technical reason why your phone number must come from the same telco as your SIM card and data plan. In fact, that’s what I’ve been doing for the last 20 years or so especially when travelling overseas (thanks to built-in SIP in Symbian phones and the marvelous N900).
Dodging roaming charges was a necessity back then as answering a call could basically bankrupt a person within minutes 
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Concerning roaming in a different country this is true. But speaking of Europe (read: Germany) you are not allowed to book into the network of Telekom with a Vodafone card of the same country and vice versa and so on.
SIM from other countries can of course.
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???
afaik there is only a built-in account in SFOS, thats all. Some more libraries to be installed but no possibility to dial out, neither as GUI nor from CLI
I do know about the relative new app S1P but it does not work for my SIP providers (linphone, Fritz)
So could you please tell us what and how do you mean with above quote?
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