I'm going with Taoman's answers.
1) I just now edited one of my working OBi / Callcentric configurations. I intentionally entered the wrong SIP password, and let it reboot the device. This resulted in the dashboard showing the 403 error. I then corrected the SIP password, and it went back to normal operation. Taoman's point was: Your Callcentric account has a minimum of two potentially different passwords: the password used by you to access your account details on their web portal, and a completely independent password you enter on their portal for your SIP devices to use. It's likely that you are using the wrong password on the OBi device -- use the "Phone password" you need to define on CC settings, on your Extensions page. Change it there, then enter the same password on the "Callcentric password" field on the OBiTALK portal page setup for the SPx. See attached. Of course, you also need to enter the correct CC number as 1777xxxxxxx
2) You can prove this is, or is not, the problem by using any softphone of your choosing (the free one provided by CC, as Taoman said, or X-Lite or whatever), to confirm that your credentials work.