I don't think SMS senders can actually tell the difference between Google Voice and other VoIP providers.

Twilio has a lookup API, which returns the subscriber name and carrier.

Here's an example response (subscriber name redacted):

  {
    "data": {
      "name": "LASTNAME, FIRSTNAME",
      "line_provider": "Google/Bandwidth.com (SVR)",
      "carrier": "Bandwidth.com",
      "line_type": "landline"
    }
  }

Ah, I always assumed Google uses Bandwidth.com completely transparently – I wasn't aware there's a separate level of "line provider" look-up available. Thank you!

Yet Facebook won’t let me sign into WhatsApp using my GV number alone.

There must be something unique about my GV number. It's even allowed on WhatsApp (knock on wood).

I registered it about 13 years ago. I didn't transfer it from a landline/cell phone, it was picked from a list of Google Voice numbers available in my area code. I've never had Fi.