I agree that trying to use Google Voice for a business is probably not the best idea.
I recently set up a system for a local limo service. They wanted 2-line rollover and the ability to forward calls to a cellphone if their internet went down.
They have an inbound number at
Voip.Ms but many other providers would work.
I configured 2 sub-accounts for him (you would have 4). The inbound number goes to the first sub-account. If that number is busy, the call goes to the second sub-account. His Obi220 has SP1 configured to his first sub-account and Sp2 is configured to SP2. In his case, he has a 2-line phone with SP1 on line 1 and SP2 on line 2.
All outbound sub-accounts have the same phone number for outbound calls.
If the adapters are offline, calls go to the configured cellphone number.
Voicemail is emailed to their email account.
If desired, you can insert a recording with your hours or send calls immediately to voicemail after hours.
You can block inbound telemarketers with NOMOROBO for free.
The 200 and the 1062 can be configured to do this rather than with Google Voice.
If it were me, I'd create the account at
Voip.Ms and buy a temporary number, set things up and test it. Then once I was happy with the setup, port in my old number and delete the temporary number. During the test, the old number could be forwarded to the temporary number. With
Voip.Ms, a temporary number might cost 40 cents to buy and $1 a month. Porting in is currently free.
While Google Voice is free, the set up I mentioned is perhaps a few dollars a month depending upon calls ($1 a month for the number and perhaps a penny a minute for calls). You can best calculate charges based upon your call volume. Calls are billed inbound or outbound.
Others here can comment on how to set up with Callcentric or other services and what costs might be, but they are probably similar.
These setups are supportable and not at the whim of Google (who says you should not use their service for business anyway).