Depending on the model of 'phone you have, it may be possible to manually block certain prefix codes, effectively preventing outgoing calls to numbers beginning with 08* (and/or premium rate lines, prefixed by 15*). Check your 'phone manual before you pay your provider a monthly fee; it may even be cheaper to switch 'phone, if necessary. You can buy a twin DECT answerphone from about €30 nowadays; that's six months' worth of Eircom's call barring charge.
Only problem is that your teenagers are likely to suss what you've done, so hide the manual!