Orange is defunct as a brand, EE are the most likely company that is responsible for the number range Orange once had , as Orange became part of EE before it disappeared, but as stated it’s almost certainly not going to be possible to do what you want .
If the number was last used on a BT mobile, the number must have been ported to BT , on cancellation, with landlines the number is returned to the range holder , it’s probably the same with mobile numbers but after all this time it’s unlikely even a landline operator would try to get the number back let alone a mobile operator, but it’s EE you need to be looking at , not BT .
