Can Anyone Recommend a VoIP Software Which I Can Use in My Speech Lab Class Using Headsets on the Computers?

I would really want to use a software wherein I would be able to set up my own server and configure it in a class. I would also prefer freeware or service providers to provide simple but quality VoIP services.

I'm planning just to have a voice chat connection with my student's computers in order to check them during our speech lab sessions.

Thank you very much.
Does anyone know a VoIP software capable on LAN? My class uses LAN. Thank you again!