Hi Dan:
Yes, you can port your land line number to Callcentric, if Callcentric can host the number. Google Voice doesn't care if you do that; it is simply calling a telephone number.
There are two opposite directions to call forwarding when using Google Voice. When someone calls your inbound Google Voice phone number, Google Voice will simultaneously forward their call to all of your enabled, linked/forwarding phone numbers, and it will ring those numbers for approximately 25 seconds. If one of the forwarding numbers answers the call within that interval, it "wins the race" and takes the call, either because you answered it, or that number's own voicemail grabbed the call. If no answer in that interval, then GV takes the call back to its own VM. GV is designed to be used with linked phone numbers that support a telephone network feature known as conditional call forwarding, or no-answer/busy transfer. You typically program your linked number to enable CCF to your GV number. This will send those calls back to GV VM.
Callcentric doesn't support CCF, so you'd need to set its ring interval to be at least 25 or 30 seconds.