Here you go again adding another augmentative point that you no one stated but you.
"when you receive it as an email is axiomatically false" No one went there.
You only get it as an email from google voice if you have settings forwarded to your email.
You can have google voice forward that sms to a dumb phone via sms.
Dont get hung up on the last mile part of sms. There many forms of sms that require different protocols when using plmn or pstn.