FWIW, Skype http://www.skype.com/ is also firewall-friendly & works great with the SME. I have a client in the UK that uses it all the time to talk to India & Kenya.
Skype is ok for a P2P connection.
- It's propriarty software, non GPL
- It's 1 on 1
- It's not a PBX
- It's not a business solution
other then that you can have great fun with skype, just not as a real business solution e.g. solid PBX