What is the best company for international conference calls?

I work for a not-for-profit organization. We have offices in Shanghai, Mexico, Canada and South Africa. We have a conference call with all offices around once a month. As you can imagine, it gets very expensive. We have been using Bell Canada. My managers are wary of VOIP as they have not had good luck with things like Skype/etc. So has anyone used any traditional conference calling companies other than Bell? Any that are great (ie. good international rates) or any to avoid? Thanks!