Good prices, great service, awesome call quality, and a web interface that doesn't suck. Calling 800 numbers is free and they support T.38. This is the last provider you'll need to evaluate :)

Support story: We had some site in Texas making random calls to our 800# over the course of like 2 months before we noticed- hundreds of different CLIDs. They helped us identify it, and gave us a credit for three months for our trouble.