Why couldn’t I use three ways calling / multi-conference call function? Why does it say “Failed to merge the call” when I use the conference call function?

The Three way calling / multi-conference call function requires network/SIM card support. If your SIM card isn’t activated to provide these functions by your service provider, the device will show an “Unable to merge calls” message if you try to use them. Please contact your service provider to enable multi-conference call service for your SIM card.