VoLTE calls not working - Asus Zenfone 5z

[emilio_s]

I have this phone which shows "4G calling" option and it is enabled, however, when I make/receive calls it falls back to 3g. I tested my O2 sim card in a different phone and 4G calling works as expected. Any ideas?

[Enlli]

There may be the possibility its not  compatible with O2

As the SIM works in another phone try asking in the Asus forums