To test the quality or your VoIP connection you can use some web tools that give you some information

- To test your VoIP quality and the MOS (from 1 to 5) for some places around the world

http://www.testyourvoip.com/

- To test if you have any problem in your routers/firewalls that do not let pass the VoIP communications.

http://www.bandwidth.com/tools/voipTest

- To know how many VoIP calls we can do simultaneously

http://www.talkswitch.com/voip/voip_capacity.php