Also, never had a trouble with DUAL Talk, that was my first carrier. Could make payments via credit card on the middle of the night, online.
Once you are setup you could watch your calls online, make payments and change the I.P. addreses of the gateways allowed to send calls in your account.
You could even sell service to people that have a gateway ,but no account. You open or close the access whenever you like it.
Both companies are very responsive to e-mails, troubles, etc.