if there is no data/wifi available, it won't forward text messages to the Freeup number. It also won't let you send text messages without data/wifi.
if there is no data/wifi available, it WILL forward incoming calls to the Freeup (or forwarding number).
Hope this helps.
Edit -
I am very surprised that calling works without data/wifi. In my experience, it doesn't. (Although I don't know what you mean by "it dials a unique number" - I have never tried that route).
It's a free service to sign up for, and as long as you use it to communicate between your Google Voice number and other U.S. numbers, it's totally free to place calls and send text messages. mybk-experience.onlmc-d.uno
From within the GV app, the path is Settings -> Make and receive calls -> Select Use carrier only (rather than Prefer Wi-Fi and mobile data).
On iOS, you will be prompted to call a number other than the one you actully wish to call. Essentially, the GV app dials that number then bridges the call to the number one actually wishes to dial.
On Android, you won't see the other number. The call will appear to be going directly to the number you wish to call, though the GV app is still dialing another number in the background then bridging the call.
If somebody sends a text to your GV# and you've set up GV properly to FW to your carrier#, then your native messaging app will show a weird # (thanks to GV magic) that you will then assign to that contact in a secondary field. And you'll know who it is cuz GV prefaces the message with their name if they're in your contacts. From that point on, it's fine to use the native app to call or text them and it'll appear as your GV# on their end.