I've got an idea. It would be a user friendly solution and some kind of innovation for any network.
Let's say I'm a gamer. Online gamer. I run through virtual lands, kill monsters and pick up every item they drop. From time to time I find a great stuff I can sell online (which is absolutely legal). Each time I try to sell something for real money, I need to receive a text message with some special confirmation code. Then I must use this code online. Imagine how many times i have to check my mobile, touch the screen, scroll etc., pure waste of time.
Wouldn't be much easier to read incoming messages online after logging in to giffgaff?