Conference Call (also called Merge Call) for a regular cellular voice call is not an iPhone-exclusive feature but rather a service offered by your mobile carrier. The iPhone simply provides the interface to use it when the carrier supports it. On the other hand, FaceTime Audio and FaceTime Video calls are Apple’s own features and do not depend on your carrier’s conference calling service.
It is also important to note that if you are trying to merge calls from two different service providers, the success of the conference call depends on whether both carriers support merging. If even one of the carriers involved does not allow call conferencing, the feature may not work as expected.
Additionally, if your device has a Mobile Device Management (MDM) profile installed by your enterprise or school, certain calling features—including conference or merged calls—may be restricted. In such cases, you will need to get in touch with your system administrator for clarification or assistance.