I can install, configure & support the following:
This can be located on your premises, or I can arrange hosting at several ISPs.
OpenBSD makes a sound foundation for running a voip server. It has a great security track record, and is more than fast enough even on moderate hardware.
Asterisk is becoming the most widely used PBX around.
trixbox is Asterisk packaged with some management tools, (like freePBX and sugarCRM), all on a single LiveCD.
If you already have a server which is relatively lightly-loaded, you can even run your PBX on the same server, using virtualization (eg qemu or VMWare). This will let you try out VOIP without any initial investment, and allow you to migrate to a dedicated server as you needs grow.