1. Open the PHONE app on your iPhone
  2. Dial the first participant's number.
  3. Once a participant is on the line, tap Add Call and to dial another participant's number. Once you dial the second number, the first call is put on hold. If you want, you can talk on the second line privately before merging the calls.
  4. When you are ready to merge calls, tap Merge Calls. The calls are merged onto one line and everyone can hear each other.
  5. If there are additional participants, they can now call in and you can merge the calls or if you want to dial out, repeat steps two and three to add more calls to the conference.



Credits:

Modified from: https://support.apple.com/kb/TA38608?locale=en_US&viewlocale=en_US