Despite the benefits of VoLTE and its predominance on the market, it will be seriously improbable that the switch will shutdown the 2G or 3G voice systems, both and much probably the 2G will remain.
In only two countries, Hungary and Italy, did the regulators become involved in whether 2G/3G networks should be switched off.
It means that in every other EU countries regulators simply ignored the matter and left the private telecom companies follow their agenda to deliver a better service (VoLTE) and saving money to not having to deal with maintenance on 2G/3G networks.
Until the day they will find out that the range of new 4/5/6G is gradually reduced (low power, high density cells, large bandwidth due to cabling) and this means that in case of natural catastrophe (or terrorist attack or war) affecting a large area such “fined grained, high performances” infrastructures will be not able to service the inner area but just working on the tiny borders, leaving all the people in such area cut-down from every mobile communications apart satellite phones service.
More or less is the same story of the wind turbines in Germany: 20 years €400bn of investment and then back to burning lignite at first international crises. VoLTE is necessary only in those countries that are going to implement stupid moves. That’s all.
Now, it is fine you say I need VoLTE because the market push me stronger towards that destination. Then, the market push you also towards Android or Apple iOS. Follow the market, then.
Finally, SFOS has the support for VoLTE. It is in beta testing like CLAT for IPv6 network operators only, and many others components. Technically speaking the whole SailFish OS is a beta and we are the beta-tester for their automotive proprietary OS. 
Then you should not even answer to a bot. 