Could you possibly help how to identify VoLTE capable devices based on UE Capability Information. As far as I know FGI bit 3,7 and 27 should be set to 1. Is there 100% working way to detect VoLTE caabilities of the UE? Can be UE usage and Voice domain preferences used for this?
No, VoLTE support is not really limited by LTE bands.
For 4G handsets, VoLTE is mostly limited by the whitelist in the handset software. If your PLMN is not in the whitelist of the handset, then VoLTE capable handset attached to 4G on your network will still refuse to use VoLTE with your network. And make no mistake, the whitelist is short.
However, it does seem that there is hope with Apple. With updated iOS and iPhone 7 and higher, the user can actually enable VoLTE in the settings - even if the PLMN of the network is not in the handset whitelist.
If any mobile or UE don’t have Volte it can be functioned by updating a firmware in mobile and set some details of networks like PLMN(MCC and MNC), modem, Apn Name, APN Type, Apn protocol and have to write volte.