Facetime - Audio only conference calls ?

I keep failing at the final step of creating a true conference call.

How To Hold A Successful Conference Call On Your iPhone

Working with WIFI and other iphone users, I press “add call” and the additional contact name is added to the top of my phone.We all have lastest ios.

However even when pressing the “merge call” button the original contact is then put & remains on "hold".

I can use “switch” to switch callers but i cant seem to enable a true conference call ie so that all participants are able to hear each other and participate.

Im sure its very obvious but what am i doing wrong ?

Any advice much appreciated.