If you are going to use the portal to configure Google Voice, and you are also a Callcentric user, you might as well use the portal to configure Callcentric, too. The portal will auto-populate the correct configuration for use with Callcentric.
Click, for example, SP2. Then, scroll down the page toward the bottom, and click "OBiTALK Compatible Service Providers". Callcentric will be on the list on the next page. Click it, enter your credentials, submit, and you're done.