If this is for people who "only have access to the internet through their phone" why do they need a topup card, when they can bookmark the website topup page or use the app?
If this was for people who don't have smart phones and don't have any form of internet access, it might make a bit more sense. Unfortunately those people aren't going to be able to vote...
Considering that giffgaff does not seem interested in doing this by automated phone, I think this may be a good alternative. There, in my opinion, should be an option which one can use in order to top up outside of using the internet.
We have been asking for a while and gg said it's not for them