I enabled the VoLTE button by *#*#86583#*#* code and activate it on 4 Xiaomi phones.
The VoLTE is also activated on my 4G SIM card by the Mobile provider (Vivacom - Bulgaria). VoLTE function was tested and work on R5+ and RN5, both phones with last MIUI Global stable ROM, but it not work on Mi5 and R3s which are with last MIUI by Xiaomi.eu, beta.
How to activate VoLTE on phone, running on MIUI by Xiaomi.eu?
I am waiting for your ideas?
Mi5 doesn't have all European LTE bands, so it might not be compatible. 4G+ and VoLTE not available for me either, in Romania. Wasn't available in Bulgaria either, when I was there 2 weeks ago. MIUI10 .eu ROM.
It does work in LTE mode, but I think it should support LTE+/4G+, connecting to 2 or more bands simultaneously for VoLte to work, not only one(normal LTE). I used A Huawei P10 for a short time and that connected to 4G+ and had VoLte and VoWIFI available for my carrier.
Not sure if 4G+ is needed for VoLTE to work, though. Sadly there are very few carriers in Europe with which M5 can get 4G+ because it does not have the very common(here) band 20...