Yes – I think that’s the only way it can work with Viewranger’s OS maps – they are downloaded onto your phone, so you don’t need any reception at all to view them when out and about. I have been known to download new tiles when out and about, but usually I get them sorted for an area before I leave – much faster over Wifi where I live.
You can also record your track, or follow a route that you’ve downloaded from them (or elsewhere). When you get back into a reception area (or somewhere with Wifi), you can sync your tracks back to their website and look at where you’ve been on their website.
I think you can also use online maps within the app (like Google’s, etc), but it’s not something I’ve ever really used.
Try downloading the Viewranger app from the Play Store (I think it’s free) – from memory you get a few free tiles to play with, so you can test it out.