OK, making progress, perhaps, or maybe not!
This is important, since you have multiple, different Gmail accounts: is it safe to assume that your account provisioned on SP1 is a consumer
Gmail.com account, and not a G Suite domain? As of now, you can no longer use G Suite with the OBi 200 series.
If this is, in fact, a consumer Gmail account, then ensure you are only signed into that one Gmail account by doing this: if you're using Google Chrome Browser, press Ctrl-Shift-N to open an "Incognito" browsing window, or, if you're using Firefox, press Ctrl-Shift-P to open a "Private" browsing window.
Go to the Google Voice settings page. Delete
all, yes, every single device and linked phone number. You should now only see "Web" in the devices section. Press Ctrl-T to open another tab in this incognito/private window. Sign into your OBiTALK dashboard. Delete the SP1 configuration (trash-can it). Wait a while for the device to remotely reboot and settle down.
Now, click SP1 and start from the beginning, to configure the same Gmail account as you are using in the other browser tab. Do not add any other devices or phone numbers to this Google Voice configuration yet. After the configuration completes, confirm that the SP1 status is "Registered". Try calling that Google Voice number from some unrelated number, not from another Google Voice number. What happens?
If it now works correctly, then go to your mobile phone, and assuming it is either an Android or iPhone running the official Google Voice app, set up the app to use that Gmail account. Then, you must program Conditional Call Forwarding on your mobile number, according to your particular carrier's star (*) codes. If you don't know how, tell me the carrier. Repeat the test call. It should continue to work properly, with unanswered or busy calls going to your Google Voice voicemail.