When calling my mum & dad’s number with the 1572 facility, my call is directed straight to voicemail.
Is there an option, that my call is recognised and then directed straight to my mum & dad to answer my call ?
Are you sure you're being directed to 1572 Voicemail? If so, then your parents have accidentally added you to their block list. If this is the case then they'll have to login to the Web portal and delete you from the list.