sipXecs is another option that has a built in conference server. I've deployed just the conference server of sipXecs to supplement a Cisco Call Manger Express customer that needed more robust conferencing services. It is all GUI via a web browser.

You could pair this with a SIP service such as 1-voip that will give you unlimited incoming minutes with unlimited concurrent calls for a very reasonable rate. You could probably purchase additional virtual numbers to meet the requirement for local numbers in various areas.