Some devices like Gigaset GS290 support VolTe but OS’s don’t. I have read that T-Mobile will stop making use of UMTS and will use VolTe instead. Does anyone know if this is true? What will this mean for alternative OS’s ?
Regain your privacy! Adopt /e/ the unGoogled mobile OS and online services
I also think voLTE is the future and UMTS will at some point in time be turned off. This will probably take some time but nonetheless /e/ should start supporting voLTE 
Does someone know if voLTE support is being worked on and can be expected in the next year or so?
I am asking because I would like to recommend the /e/-Store to relatives and they are asking for voLTE because of the better call quality.
VoLTE is already in some devices: specifically, I know that it is present, and it works, in the most recent (and unfortunately the last for now) official build for Sony Xperia XZ1 Compact (lilac). Whether or not it appears and works in a specific device will (I think) depend on whether a: the device supports it, and b: whether your network operator supports it.
I think you will therefore need to check on a per-device basis whether it is or will be in future supported for a specific device
To check, try Settings > Network and Internet > Mobile Network. If the phone supports it, there should be a “VoLTE” switch. If the phone supports it but your operator does not, then the swicth is likely to be greyed out and not ‘clickable’
In particular I am interested in the /e/-GS290 and on https://esolutions.shop/shop/e-os-gs290/ is says that “VoLTE Not yet supported by /e/OS on this phone” so there also seems to be c: /e/OS does not support VoLTE. As @Kea mentioned the hardware should support VoLTE.
I know someone who talked to the customer support of TCL about VoLTE and was told that a phone “model” also has to be registered with the network operator (Telekom in this case).
As @petefoth noted above, VoLTE is already present in some devices, but “Official” VoLTE and Wi-Fi Calling support are not scheduled for /e/OS until version 0.16 milestone per devs. I don’t have the link handy, but this info came straight from one of them on Gitlab.
Ah ok on Gitlab the is more activity regarding these issues. Thanks for pointing out.
general VoLTE /e/ Issue:
Samsung Galaxy Issue:
Gigaset GS290:
Issue #2494 (only 2 links per post)
I can confirm that VoLTE and VoWifi definitely works with Telekom (T-Mobile Germany) and GS290 stock rom, but /e/ doesn’t support it yet. I don’t have a Gitlab account, so i’ll add my 
here.
Have you any idea why this is? Is it because of privacy? sailfish doesn’t support it either.
/e/ dev team is working on making VoLTE possible on the OS. There are a number of open issues on this subject in the Gitlab
It is also one of the targets we have set to achieve in 2021
T-Mobile is sending me Texts that my service will be “disrupted” if I do not install their patch. Since I am certain their patch will destroy all privacy security, I do not want to touch it.
I have a Samsung Galaxy SM-G960F from e-foundation.
I have received emails of that nature from T-Mobile but apparently only for one of my devices (Moto G5 Plus potter) running Nougat ROMs.
Still trying to figure out what to do there before July. The device has custom ROMs up to Android 11 (though 8 was the last stock available) but there’s no guarantee moving up will change the situation.
I went ahead and ordered theTeraCube Murena Teracube 2e - Murena - deGoogled phones and services
which says it has VoLTE. We will see.
I have the T2e. From what I can tell/see it does have VoLTE. What it doesn’t seem to have is VoWiFi (WiFi Calling).
We’ll see what happens with that.