Yes, I think that's the way it's done - or by dialling a special number connected to a service that reads your button presses directly (that's how Telia does it if you get the code from for instance a teller machine). I'm a bit sketchy on the details as I've never had one of these myself - I've always been on a monthly basic rate (then pay off anything over that basic the following month) and currently my employer covers my phone.

/Michael