018 is not really free. You would have already paid for it in advance in the prepaid and post paid plans. Besides, 018 is using voip. Quality is not all that fantastic during peak times. As for 008, they can substitute with voip, too, but via the more stable routing.
Still, making idd calls in today's context is dirt cheap compared to the time when Singtel was the monopoly. Do you remember paying more than $3 per minute? Can you imagine how much money was generated those days.