If you don't have a smartphone/internet access - please contact the sender (e.g. school, local authority or charitable organisation) to ask if they can provide an alternative, such as a printed version.
If you have a smartphone but no data - please access the voucher on your phone whilst connected to wifi then head out with it already on your screen. To be extra certain, take a screenshot of the voucher's contents. It should still scan just fine. Don't forget, you can also print your voucher.