In case anyone else stumbles over this. Callcentric requires two different settings then the defaults of an N900. Both settings are under Advanced settings. The transport should be UDP (not Auto) and "Loose routing" should be checked. Otherwise, the settings I used were:
Address: 1777MYCID@callcentric.com
Password: ********** (My password)
Advanced settings:
Username: 1777MYCID
Transport: UDP
Outbound proxy: callcentric.com
Loose routing: checked