I am unable to merge call/conference call from my iphone 12, VOLTE is off

You can leave VoLTE ON or OFF and that should not affect whether you can add / conference another person into your conversation.

I recommend that you contact your cellular service provider and ask them why you are not able to add people to your conversations. They control this feature so they should have a fix for you.

Axel F.

Your last sentence concerns me. If your iPhone gets too hot to touch (You said it burned your hand) then you should have it examined at the Apple Store or at the Authorized Service Center.

You can quickly try the following as well:

Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

Axel F.