I get that frequently, even in the middle of the day for my pay as you go account. However, sometimes going round the menus in a different way gets round it. Odd. Seems to happen most often when I'm due to refill, I keep wondering if it's a deliberate "frustration" to get you to forget to refill on time and lose your banked time.... like how they seem to send reminder texts reliably for 4 months in a row, then skip a couple of months...