Wow, this is two pages of trying to use a hammer to drive in a screw. You might eventually succeed, but all parties involved will be sore at the end.
Let me suggest an alternative. Don't use Google Voice at all for your work solution.
I think the original discussion went off the rails because it didn't address your actual job requirement.
As I understand it, you have one of those work-at-home jobs, wherein you are required to get a conventional inbound telephone line, with no call forwarding or voicemail or three-way calling at all. The employing company then forwards inbound calls to your personal phone number, one at a time, and you do your thing with that customer, until you are done. On your company's side, they use ACD (automated call distribution) software to manage your call queue for you. When your phone is no longer busy, it sends you your next chunk of work (your next caller).
Right?
If so, do this: get a free New York inbound phone number (DID) from Callcentric. Do not set up any calling features at all on that number. It's just going to be a plain phone number that rings your OBi-attached phone. When nobody answers, it keeps ringing, and when it's busy, it generates the correct busy signal. Your company's ACD needs a carrier-supplied busy signal; fake, recorded hocus-pocus won't work, because your work's switch knows that your carrier answered the call. They need to get the busy signal instead of an answered call, not something that sounds like a busy signal.
Factory reset your OBi box to remove that spaghetti-pile of settings you may have added. Use the OBiTALK portal to delete and then re-add your OBi device. Then, use the portal page's bottom-most link to configure service with other providers, and then select Callcentric. Give the portal your Callcentric free DID account number (the CC 1777nnnnnnn account number, NOT the DID telephone number) and the SIP password you set up on the CC web site. Call the CC telephone number from some other phone number, to test that it rings or goes busy as I descibed. Once you get it working, give the CC DID telephone number (the New York number, not the 1777 number) to your employer. That's it.
Optionally:
IF you would also like to use Google Voice, you can configure one of your OBi's other service provider slots with GV, using the portal. Just do NOT use that number for work calls.