Question:
Is there any open-source or relatively cheap VOIP conference call software that I can install on a server (either sun/solaris or windows) that will allow people to dial in to from their PCs?

Answer:
Then you can get them to dial 1-222-XXX-XXXX and it is set up for conference calling. Not 100% secure as anyone dialing the same number will be able to join in but its highly unlikely. Works fine when I use it and it isn't limited to a set amount of people.

iChat for Tiger allows you to participate in audio conferences with up to 10 participants. High-quality audio compression techniques ensure crystal-clear conversations while full-duplex sound lets everyone speak naturally.

The XTEN-Pro softphone will bridge 7 lines - XTEN have spun off their software but you should find links on their site () - all your users should be able to get away with the free version of xten., you using pro for the bridge.