As we revamp up our own and a client's voip services, it seems incumbent to say a few words here.  A nice post on what we've found for VOIP/SIP termination can be found here: http://www.trixbox.org/forums/trixbox-forums/open-discussion/recommended-voip-providers .  As the post in-part states, we use Vitelity for its overall high-uptime service, quality Web-GUI for configuration, and reasonable rates.  Another one I keep hearing good things about is http://connect.voicepulse.com/, but their pricing structure is a bit more confusing and their PSTN termination rates are higher.