Sometimes on VoLTE operator, the VoLTE is not working.
I had this problem on Xiaomi Redmi 9 with indonesian ROM (installed for MIUI native call and message apps instead of google ones).
You can solve this problem adding another APN named IMS. This APN must be added only and not selected as active.
So if VoLTE is not working follow these steps:
- Call: *#*#86583#*#* (enables VoLTE flag in configuration)
- Add this APN without selecting it:
- Nome: IMS
- APN: ims
- APN type: ims,ia
- APN protocol: the same of the active APN
- APN roaming protocol: the same of the active APN
- change only these parameters, leave the other params unchanged.
NOTE: perhaps this problem is due to the fact that this APN in the EU ROM is already present, but is hidden.