With google voice you would be calling using their app so there should be no Thai voice. The calling would be dialed exactly as if you were in the US like 555-123-4567. No need for a VPN. AIS would only see data and not a phone call
You can apply for an O-A visa in your home country which is good for a year or more with foreign travel. The only drawback is that you would need to purchase a specific health insurance. At the end of the O-A you can switch to an O with no insurance.