Voip server for teaching project


I search a VOIP server for a teaching project. Basically it will be conference call between 7 people (max). I read some documents on the VOIP (mainly on voip-info.org) but I’m little confused.

Typically, the server on a scheduled time (the schedule is in a database) will call the teacher and the students (their numbers (sip ?) will be also in the database). Or the teacher create a conference and will be called. So I presume that I need a server with some API available (java or c# will be the programming language). The audio quality is a critical requirement (I can buy some codec licence). Also I don’t know how the bandwidth is used : which caller’s bandwidth is mostly used (the first ?), if a “generated” user on the server create the conference call it will be the server’s bandwidth that will used ?

I plan to use gizmoproject softphone for the client-side, I read that it can be used with asterisk, isn’t it ?

Thanks for your advices !