If you want to route your Google Voice number's inbound calls to a Callcentric phone number, then that CC phone number will be receiving voice telephone calls on your attached phone. If you also plug in a fax machine, you need to either set it to not auto-answer inbound calls, or you need to get another (second) Callcentric number, and dedicate that number to your fax machine. In so doing, you'd set the ring pattern for the second number to be a "distinctive ring" pattern, and your fax machine would have to be able to properly handle answering only on the distinctive ring.
Callcentric also has an inbound fax mailbox service, which can receive inbound faxes and email you the document as a file.
As for outbound fax, sometimes it's flaky, due to VoIP technology vs. fax technology, but you could fax from either the GV number or the CC number.
