In Google Voice, do all SMS messages have to be transmitted via a data connection?
If you are using the Google Voice or Hangouts app to send or receive messages using your Google Voice number, then yes it must have a working data connection, but it's data usage will be extremely minimal. 

In Google Voice you can setup your SMS messages to forward to email or your actual carrier's SMS service if you prefer. You can replay from either email or carrier SMS via both methods as well.