Why can’t I do a 3 way call on iPhone?

Apple advises that conference calls (merging calls) may not be available if you are using VoLTE (Voice over LTE). If VoLTE is currently enabled, then it may help to turn it off: Go to: Settings > Mobile / Cellular > Mobile / Cellular Data Options > Enable LTE – turn Off or Data Only.

How do you make a 3 way phone call?

To start a 3-way call on most smartphones:
  1. Call the first phone number and wait for the person to answer.
  2. Tap Add call.
  3. Call the second person. Note: The original call will be put on hold.
  4. Tap Merge to begin your 3-way call.