" ... but that there is no verification they are not Google Voice numbers ..."

This isn't unknowable ... why don't you just check ?

  curl -s -X GET "https://lookups.twilio.com/v1/PhoneNumbers/$number?Type=carrier&Type=caller-name" -u $accountsid:$authtoken | /usr/local/bin/jq '.'

... or, if you're non-technical, just use mycarrierlookup.tv or checkcarrier.estonia or whatever ... there are 20 or more such sites extant.