Just search on this forum MIA Sphere.
Set your premium SMS spend to zero and contact Optus to ask for a refund.
I would start on online chat make a fuss and request them to ask for the charges back from Mia sphere. Keep on at them and ask them for which number to ring. It will take time I'm afraid.
More importantly go into my account and set your premium SMS spend to zero and also request this on online chat. This will prevent future charges.