FWIW, I have an N900, and only a callcentric "trial" account (I can only call other callcentric numbers) but I used the help sections on callcentric's site, and the settings for "bring your own device" (or something to that effect). I can't remember if I had to use an online trunk to support it though.

Some things to try, if you already have not:

- enter the username in different ways, e.g. 1234567899, or 1234567899@callcentric.com, etc

- set up a trunk somewhere, based on callcentric settings (there is a step-by-step if you google "sipdroid for Android", with info regarding pbxes.org )