I am probably one of the few people on the planet who uses ordinary conference calling quite a lot. When in a call with someone it is sometimes helpful to add a third person to the conversation. This is done by putting the other party on hold while the call is established to the third person. Once the third person picks up you have to explain that you have someone on the other line and would like to make a conference that person in. In most cases that explanation takes many additional seconds during which the other side is still on hold. In other words, this solution is far from ideal in practice.
From a usability point of view it would be much better if the conference call is already established while the third party is called. The person on the other side could hear what is going on, you can talk to the person while it is ringing and the connection between all parties would be instantaneous once the third party accepts the call. No long explanation, just a quick note that XYZ is also on the line which many people are used to that get calls from people in a meeting room.
Unfortunately that’s not how conference calls are implemented in legacy circuit switched and (even) in IMS telephony today. And I have no illusions, I don’t think we’ll ever see this implemented there. But perhaps, who knows, it will happen in telephony and conference calling systems that are still evolving. Would be nice.