Callcentric (like may SIP providers) has a DNS SRV record under _sip._udp.callcentric.com . The auto-prefix option tells the OBi to add the _sip._udp. to the domain name before looking it up. In the case of CC, the record specifies that the client connect to port 5080, which is of course different from the SIP default of 5060.
For most consumer routers, connecting to 5080 is a good thing, because arbitrary outbound ports are open (by default), yet avoiding 5060 means that we bypass any ALG listening there; ALGs in consumer devices generally cause far more problems than they fix.
In your case, the firewall by default (or with your choice of settings) may be more restrictive, and that's causing the registration to fail.
You have had more trouble with this issue than 99% of users. To understand what's going on, please post make/model/version of your router (and modem, too), along with any special settings you use, and any other VoIP devices or apps on your network (that e.g. might be using the same ports).
