That is just one of many threads that get started almost daily by newbies thinking that just because they have inserted the sim into their phones and got a signal and in most cases received a text from 2020 with internet settings in it, then the sim has been activated.
It is then only after about 10 replies that the newbie then makes a slip up and informs us that they have not topped up yet that we realize they HAVE NOT yet activated.
So what I propose is this.
All new sim cards once inserted into a phone send out a signal to giffgaff informing them the sim card has a signal.
Then giffgaff/O2 send out the internet settings 2020 text.
As well as that text why not send out another text saying:
"Your sim card is not yet fully active. To activate visit http://www.giffgaff.com and click the activate a sim link. Remember during the activation process you will be required to to up minimum £10"