How do I add a third person to a call I didn’t originate?

Hello ddheagerty,

Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are trying to add another person to a phone call on your iPhone. This is possible in specific situations, so please see this Apple resource to see if this is possible in your situation:

"With GSM, you can set up a conference call with up to five people (depending on your carrier).

Note: Conference calls may not be available if your call is using VoLTE (Voice over LTE) or Wi-Fi Calling.

1. While on a call, tap Add Call, make another call, then Tap Merge Calls.
2. Repeat to add more people to the conference.
3. During the conference call, do any of the following:

• • Talk privately with one person: Tap , then tap Private next to the person. Tap Merge Calls to resume the conference.
  • Add an incoming caller on the same line: Tap Hold Call + Answer, then tap Merge Calls.
  • Drop one person: Tap  next to a person, then tap End."

While on a call on iPhone

Cheers!