> Recently I changed back to Telstra.
>
> As I don't use voice mail, etc Telstra offer a free missed call service
>
> Does anyone know of a way of turning this off too?
>
> Or is it a call Telstra?

Haha
Call them and tell them to turn it off, which they will gladly do for you.
BUT, in about a month or so it will be on again. Been there, done that.