Unfortunately, giffgaff does not support conference calling, so you can't start or manage a conference call using your giffgaff service.
If someone on another network starts the conference call, you can still join as normal.
Alternatively, you can use a third-party App such as WhatsApp, Whypay or Zoom, that offers conference calling over WiFi or mobile data.
Did this solve your problem? If not, pop over to our friendly community or chat with our giffgaff HelpBot for a quick answer.