THIS DID NOT Solve my problem.
My carrier, Verizon, confirmed that conferencing multiple calls (up to 5) is supported by my plan. In fact, their instructions for how to do this on my iPhone 7 Plus are exactly the same as Apple’s instructions. I was able to make this work only once.
AFter I call the first caller, I press the + to add a caller. I see the original caller placed on HOLD. After I dial the second caller and successfully connect, I press the “merge calls” button. Nothing happens for a few seconds. Then the “HOLD” button is lit. If I wait a bit, without touching the HOLD button, the call gets disconnected. Similarly, if I press the HOLD button, I also lose both callers. Now what? I have the most current IOS, by the way.