Hi Greg:
There is nothing magic involved. Simply configure your desired Google Voice accounts on SP1--SP4, using the OBiTALK portal dashboard. When doing that, it will have check-boxes on the forms to select which SP rings which physical phone line jack for inbound calls, and which SP to use as the default for outbound calls when you pick up the phone.
Disclaimers:
- You should also subscribe to E911 service, since GV cannot make emergency calls.
- Google Voice is not intended for business use, and obtaining multiple Google Voice numbers may be considered abuse of the service. Google can shut down user accounts without warning if its automation detects commercial use patterns, e.g., many short-duration calls during a given time period, or other behavior suggesting a call center.