Was that conference call business related? And depends on that conference phone number that you are calling into.
Google Voice service has limitations on conference calls, or will not allow calling into certain free conference calling areas because of the high-cost of traffic pumping calls.
Try making that same conference call within your Gmail email inbox using Chrome browser, at the Google hangouts (bottom-left of screen) and see if the connections are allowed and/or how long.
For Google Voice service, especially for conference calling, these are called "traffic pumping" numbers and have very high cost of terminations.
Google Voice will not connect to these type of numbers. If you have a conference number with 800 toll free numbers, it will work with GV.