There is no such thing as "FREE" conference calling. Somebody has to pay for it. "Freeconference.com" has to make money somehow. So, "Freeconference.com" is scamming other telephone carriers, via a practice known as "access stimulation" or "traffic pumping." In this scam, the conference company makes an arrangement with a small, rural telephone company that is authorized to charge very high interconnection rates to other telephone companies which connect to their telephone numbers. The rural telco then shares the revenue from these exorbitantly-high-rate calls with the conference calling company. It's unlawful under FCC regulations.
You can read more about it here:
https://www.fcc.gov/general/traffic-pumpingGoogle Voice used to block these calls. Now, as an accommodation to its users, Google Voice charges one cent per minute to call these numbers. Your account doesn't have any calling credit on it, so the calls fail. You either need to add calling credits, or not use the conference calling service with Google Voice.