I've been asked several times for a recommendation on conference call providers. We've only done preliminary research on conference call services, so I might not have all the information, but I feel good about recommending Dimdim Pro. It supports up to 50 attendess, integrates with Microsoft Office and has some cool features, like the option to record meetings. Also, this is an open source software, so that means the end-users (like you) change, improve and redistribute the software. This cuts down on the overall development costs, and is a big reason Dimdim can afford to charge $19/month or $228/yr on their annual Pro plan. Keep in mind a downside to Dimdim is that it doesn't offer some premium audio/teleconferencing features, such as audio streaming broadcast and call-in/call-back. My suggestion would be to sign up for their free trial, Dimdim Free, and see if their service is going to meet your needs. (Note: Their free trial only supports up to 20 attendees).