I used a search for “VoLTE” and found this
Yes, I would be happy to let you know what I find. The whole idea behind buying a Xperia 10 III is to try SFOS with VoLTE turn on.
VoLTE works flawlessly with Vodafone Germany but 5G is sort of suspicious. I’ll keep testing. There were cases where I switched off VoLTE while roaming esp. in rural areas in Europe. But most of the time it just worked.If it happens to fail you won’t actually notice anything except when checking the settings.
In Italy, there is no way to get VoLTE working.
You won’t even have data working at all with Xperia 10 III with Vodafone and WindTre networks.
Sadly I haven’t been to Italy since summer 2021. (Shame on me!)
Roaming with my Vodafone SIM worked well AFAIR.
But esp. VoLTE seems to be very provider specific and support seems to vary from device to device in different areas. Europe, they said, universal roaming, they said… 
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A while ago dcaliste shed some light into the inner workings of VoLTE on SFOS and specifically the issues which lead into this inconsistent experience across network operators. Here’s the link to his summary on the topic, which I think is good reading for anyone interested in the matter.
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Assuming it works well in Finland, too -
Chances are it will work just as well in all Nordic countries (they have some sort of collaboration wrt mobile networking going on)
VoLTE is working fine here (KPN, the Netherlands), but I’m also having odd issues on 5G: My phone often stays connected to 4G despite 5G being available; and when “Prefer 5G” is selected, call recording often doesn’t work from what I’ve tried.
All is fine as long as “Prefer 4G” is selected for me!
I recently went down the rabbithole on a friend’s 5G router. It’s in the countryside and I was suspicious if 5G is even available, as the salesperson claimed.
There are no 5G towers around there. But.
Turns out there is a special mode where the device connects to 2 4G towers, and it’s still called 5G. However, there was only 1 (4G) tower available and it was a little unclear if the device then still claims a 5G connection becauseit potentially could connect to a 2nd tower.
So, it could just be an issue of reporting the connection differently. At least some of the time.
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5G standalone or non-standalone?
The preference is presumably about standalone, whereas non-standalone is more of a “if you need it” sort of thing.
I think some phones cheat quite a lot with the technology indicator - though i suppose SFOS would follow that from Android to some extent.
All three masts in my vicinity are Huawei RRU5815’s, they were all placed there last month, but they support both SA and NSA…, other than that it says “Transmission: Glass. Has an RRU5815 in each sector for 8T8R 3500MHz. 1.5m 2L5H antenna with single set of cluster ports.” does that give you a clue? I’m more familiar with WiFi than mobile 
And yeah I read about the cheating thing regarding what icon to show in the status bar and for providers to promote their networks as 5G where really it’s more 4G still… well, I rarely use mobile for data, and I know 5G is supposed to consume more battery, so I’m happy to leave it at 4G if it solves my issues and still allows VoLTE!
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Supports does not equal has configured. I’d be surprised if there was a single radio that did not support both - the distinction happens higher up the stack. So not much to go on at all really.
The power consumption is always a problem initially, before the optimizations roll around. 5G is at least designed to be more lean, i.e. with less mandatory overhead.
Indeed… so, to answer your original question: I suppose either the network is non-standalone; and the phone falls back to 4G because 5G is not needed at that point and all is working as intended, except the issue with call-recording; though that’s probably anecdotal on my end.
Or the network is either SA or NSA, but the phone or OS has some (compatibility-) issues and is swapping network modes randomly because of it! 
Czech Republic: O2, Xperia 10 III
For quite some time now (I very rougly estimate one year) the only way to have working calls is with VoLTE enabled, without it the other party won’t be able to reach me and only afterwards will I receive a notofication SMS of the fact.
Unfortunatelly sending SMS works only with VoLTE disabled which is frustrating and embarassing.
BTW I didn’t realize 10 III has 5G until the latest update.
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I have the same issue (with my 10iii) not being able to send SMS when VoLTE is enabled. I’m on the network of the largest provider in Denmark, Yousee.
In addition I have to other issues which I think is related to the way VoLTE is implemented.
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I receive the same SMS several times, or an download error, due to a faulty receipt acknowledgment from the phone to the sms-server. This issue is regardless of VoLTE being on or off. But the problem did not exist before VoLTE.
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Some (maybe 50%) of incoming calls fail with an error message the there is no network connection, but the network connection is there, I can call back immediately with out problems. My feeling is that even if VoLTE is off the phone does not drop the connection to 2G for some calls.
All three issues disappear when I move the same sim-card to my XA2 - which by the way work much better after upgrading to 4.6.0.13.
I have tried everything including total reflash first of Android 11 and then SFOS 4.6.0.13, nothing helps, so I have to use my old XA2 and for second year running keep the 10iii in the drawer. I have pre-ordered the C2 phone as I doubt my 10iii will ever work with SFOS.
So far I’ve been in Poland, Denmark and Sweden. An hour ago I arrived in Norway. I had two of the usual ‘missing SIM’ reboots at the danish border in and out.
Phone, SMS, mobile data, navigation: everything just works like expected.
10 III, 4.6.0.13, Vodaphone Germany
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Hi Per
I m also on the yousee net through Telmore.
I do sometimes experience that I receive the sms several times, but not that often.
The issue with the incoming calls I do not have.
The biggest issue I have, is if the GPS is on, I will get sound issues and the best here will be to restart the 10iii. So if I have to use the GPS on my 10iii, I will always restart the phone here after.
I have not seen the issue with GPS disrupting sound on my 10iii, maybe because I quickly gave up on my 10iii, since the most fundamental things like calls and sms are unreliable I could not use it.
It is a bit odd that some people seem to be able to use the 10iii and others not. My guess is that some providers use slightly different network setup, and the implementation of 10iii is not robust enough to work across all networks. I did at total reflash of 10iii, starting with reflashing Android 11, I tested the phone and everything worked fine, then flashed SFOS 4.6.0.13 on it and all the before mentioned problems were there again. I move the same sim-card to my XA2 and everything works (XA2 work much better since 4.6.0.13). So the issue must be somewhere in the way SFOS is implemented in 10iii.
I have been with Jolla since receiving my Jolla-1 January 2014, and had pretty much every phone available since then, the Aqua Fish, Xperia X, Xperia XA2, but the 10iii with SFOS is the worst phone I have ever had. But I’m sticking with Jolla and have preordered the C2 hoping this will allow me to continue using my much beloved SFOS.
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I have also been on the boat since 2014 
I have used the Jolla 1, Xperia x, experia xa2, and experia 10iii.
I have now bought 10v and waiting for the release of the SFOS.
In the beginning for the Jolla 1, there was an issue with the mms. I did try different providers and experience different issues for the different providers. So yes, the providers network are differents.
Concerning you phone call issue, have you tried to get a new SIM card?
It’s a bit strange you have this issue.
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I have not tried a new sim-card. My existing sim-card work fine on the same phone when the phone has Android 11, and it also work fine on my XA2.
when receiving some calls on the 10iii with SFOS, there is just a beep and then the display shows an error of “no network connection” but the connection is there I can call back immediately, I think SFOS, even with VoLTE off, sometimes does not correctly drop the connection to 2G, and then fail. Together with the issues of not being able to send sms with VoLTE on, and the very often multiple receipt of the same sms, or an error saying “The MMS cannot be downloaded” even there is no MMS but an sms. All together it’s so unreliable and annoying that I gave up on my 10iii.
I have considered trying a different provider, but I just don’t want to change provider whenever there is a problem with SFOS. Some years ago I had to move away from 3 when they started using some domestic-roaming that did not work on the Xperia X I was using at the time.
My provider is Unotel, which is part of the TDC group and using the Yousee network, just like your provider Telmore, I assume their network setup should be the same.