Apparently its possible but a little opaque. You have to go to LIVECHAT and ask.
Seems the main reason is if you are getting nuisance calls. If you were out of plan you could just do it any time so can't see why it would be an issue in a plan. Let us know how you get on.
Thanks for your reply Peter. I got it sorted out in the end. I had been to the optus shop in BELMONT and was told I would have to buy out my plan and start afresh. I usually avoid this shop as I’ve never had satisfactory service. I rang optus and got talking to a lady called Cher in Manila and she was great. I had to pay $50 and she gave me a choice of 4 new numbers.
So thanks again for your help.