iPhone 12 Pro max

Start with these references to make sure you have the basics down:

• How to make conference calls
• While on a phone call

If still not working: force quit the phone app and test a call. https://support.apple.com/en-us/HT201330

If still not working: power cycle your phone (turn off and on). Test.

If still not working: update the iOS to the most recent version. Test.

If still not working: back up your phone, erase it and set it up as new (not from a backup). Test.

One of these is likely to work. If not, contact Apple support.

Check with your cell service provider.

Have you contacted your cellular voice provider to see if they can assist you with this as conference calling is one of the services they would provide you.