Its unclear you exact situation. You seem to have stopped paying Optus for the internet? As far as I know Optus don't sell a phone only plan so you've essentially left Optus 100%
FYI People can't 'own' phone numbers only telcos (like Optus).
Normally you would port your number to the new ISP when you leave and then that would shut the Optus contract down. If you don't do that specifically then the number returns to Optus and they can give to whomever.